So there are two places you can get any of these from: apple, or a cached copy at edgesuite. The first part of the url is:
http://appldnld.apple.com/ (normal downloads from apple) http://pfd.apple.com/ (protected assets from apple) http://appldnld.apple.com.edgesuite.net/content.info.apple.com/ (both, cached version)
Combine one of these with the url fragments listed below to create a full link.
You can't download a protected assets link without a download key. These change often.
For firmware not based on iOS there are three files: firmware image, signature file, and documentation file. Links are given for the firmware image, and the other two are in the same location. All three are listed for the first item, and the same pattern holds throughout. For iOS based devices, signature files are dropped and documentation requires a separate link, given afterward.
iPod (Scroll wheel) and iPod (Touch wheel) (aka first and second generations) iPod/SBML/osx/bundles/061-2686.20060912.ipTsW/iPod_1.1.5.ipsw iPod/SBML/osx/bundles/061-2686.20060912.ipTsW/iPod_1.1.5.ipsw.signature iPod/SBML/osx/bundles/061-2686.20060912.ipTsW/iPodDocumentation_1.1.5.ipd
iPod (dock connector) (aka third generation) iPod/SBML/osx/bundles/061-2687.20060912.IPwdC/iPod_2.2.3.ipsw
iPod (Click Wheel) (aka monochrome fourth generation) iPod/SBML/osx/bundles/061-2691.20060912.ipDcw/iPod_4.3.1.1.ipsw iPod/SBML/osx/bundles/061-2692.20060912.pODcW/iPod_10.3.1.1.ipsw
iPod Photo and iPod with color display (aka color fourth generation) iPod/SBML/osx/bundles/061-2693.20060912.PdwCD/iPod_5.1.2.1.ipsw iPod/SBML/osx/bundles/061-2694.20060912.ipDcD/iPod_11.1.2.1.ipsw
iPod (5th generation) iPod/SBML/osx/bundles/061-2788.20061206.nS1yA/iPod_13.1.2.1.ipsw iPod/SBML/osx/bundles/061-2965.20080313.R45jT/iPod_13.1.3.ipsw iPod/SBML/osx/bundles/061-2789.20061206.9IIut/iPod_20.1.2.1.ipsw iPod/SBML/osx/bundles/061-2966.20080313.2WqrT/iPod_20.1.3.ipsw
iPod (5th generation Late 2006) iPod/SBML/osx/bundles/061-2790.20061206.iPr9t/iPod_25.1.2.1.ipsw iPod/SBML/osx/bundles/061-2967.20080313.Cnvkg/iPod_25.1.3.ipsw
iPod classic iPod/SBML/osx/bundles/061-3619.20070905.iNq3b/iPod_24.1.0.ipsw iPod/SBML/osx/bundles/061-3877.20070914.n9gGb/iPod_24.1.0.1.ipsw iPod/SBML/osx/bundles/061-3929.20071005.jGu6t/iPod_24.1.0.2.ipsw iPod/SBML/osx/bundles/061-3940.20071115.0Iun5/iPod_24.1.0.3.ipsw iPod/SBML/osx/bundles/061-4010.20080115.Ad4rF/iPod_24.1.1.ipsw iPod/SBML/osx/bundles/061-4306.20080430.Gtr54/iPod_24.1.1.2.ipsw
iPod classic (120 GB) iPod/SBML/osx/bundles/061-4962.20080909.Aaqs3/iPod_33.2.0.ipsw iPod/SBML/osx/bundles/061-5740.20081111.ZaU7Y/iPod_33.2.0.1.ipsw
iPod classic 160 GB (Late 2009) iPod/SBML/osx/bundles/061-6797.20090909.3uTfE/iPod_35.2.0.2.ipsw iPod/SBML/osx/bundles/061-7155.20090925.Ju879/iPod_35.2.0.3.ipsw iPod/SBML/osx/bundles/061-7299.20091217.Bghyt/iPod_35.2.0.4.ipsw
iPod mini iPod/SBML/osx/bundles/061-2688.20060912.iDMni/iPod_3.1.4.1.ipsw iPod/SBML/osx/bundles/061-2689.20060912.ipDmn/iPod_6.1.4.1.ipsw
iPod mini (2nd generation) iPod/SBML/osx/bundles/061-2690.20060912.PdMin/iPod_7.1.4.1.ipsw
iPod nano iPod/SBML/osx/bundles/061-2697.20061016.iPDnO/iPod_14.1.3.ipsw iPod/SBML/osx/bundles/061-3190.20070315.p0oj7/iPod_14.1.3.1.ipsw iPod/SBML/osx/bundles/061-3191.20070315.BgV6t/iPod_17.1.3.1.ipsw
iPod nano (2nd generation) iPod/SBML/osx/bundles/061-3325.20070507.KnB7v/iPod_19.1.1.3.ipsw iPod/SBML/osx/bundles/061-3326.20070507.0Pm87/iPod_29.1.1.3.ipsw
iPod nano (3rd generation) iPod/SBML/osx/bundles/061-3878.20070914.P0omB/iPod_26.1.0.1.ipsw iPod/SBML/osx/bundles/061-3930.20071005.94rVg/iPod_26.1.0.2.ipsw iPod/SBML/osx/bundles/061-3941.20071115.Hngr4/iPod_26.1.0.3.ipsw iPod/SBML/osx/bundles/061-4011.20080115.Gh5yt/iPod_26.1.1.ipsw iPod/SBML/osx/bundles/061-4276.20080430.Gbjt5/iPod_26.1.1.2.ipsw iPod/SBML/osx/bundles/061-5164.20080722.hnt3A/iPod_26.1.1.3.ipsw
iPod nano (4th generation) iPod/SBML/osx/bundles/061-4637.20080909.vfH8i/iPod_31.1.0.ipsw iPod/SBML/osx/bundles/061-5529.20080915.3ngi4/iPod_31.1.0.2.ipsw iPod/SBML/osx/bundles/061-5583.20081111.Bhyui/iPod_31.1.0.3.ipsw iPod/SBML/osx/bundles/061-5808.20090805.Fvgtr/iPod_31.1.0.4.ipsw
iPod nano (5th generation) iPod/SBML/osx/bundles/061-7165.20090909.AzPKm/iPod_1.0.1_34A10006.ipsw iPod/SBML/osx/bundles/061-7408.20091109.Kef5t/iPod_1.0.2_34A20020.ipsw
iPod nano (6th generation) iPod/SBML/osx/bundles/061-9054.20100907.VKPt5/iPod_1.0_36A00403.ipsw
iPod shuffle iPod/SBML/osx/bundles/061-2975.20061218.in8Uq/iPod_128.1.1.5.ipsw iPod/SBML/osx/bundles/061-2953.20061218.yRet5/iPod_129.1.1.5.ipsw
iPod shuffle (2nd generation) iPod/SBML/osx/bundles/061-3316.20070618.9n1bC/iPod_130.1.0.3.ipsw iPod/SBML/osx/bundles/061-4376.20080303.Bi6T9/iPod_130.1.0.4.ipsw iPod/SBML/osx/bundles/061-3317.20070618.nBh6t/iPod_131.1.0.3.ipsw iPod/SBML/osx/bundles/061-4377.20080303.fk3ir/iPod_131.1.0.4.ipsw
iPod shuffle (2nd generation Early 2008) 2 GB iPod/SBML/osx/bundles/061-4378.20080303.G5T87/iPod_133.1.0.4.ipsw
iPod Shuffle (3rd generation) iPod/SBML/osx/bundles/061-6315.20090526.AQS4R/iPod_132.1.1.ipsw
iPod Shuffle (4th generation) iPod/SBML/osx/bundles/061-8479.20100811.Cdf87/iPod_134.1.0.ipsw
iPod Touch iPod/SBML/osx/bundles/061-3882.20070910.N8uyT/iPod1,1_1.1_3A101a_Restore.ipsw iPod/SBML/osx/bundles/061-3932.20070927.p23dD/iPod1,1_1.1.1_3A110a_Restore.ipsw iPod/SBML/osx/bundles/061-4036.20071107.9g3DF/iPod1,1_1.1.2_3B48b_Restore.ipsw iPod/SBML/osx/061-4060.20080115.9Iuh5/iPod1,1_1.1.3_4A93_Restore.ipsw iPod/SBML/osx/bundles/061-4312.20080226.Btu45/iPod1,1_1.1.4_4A102_Restore.ipsw iPod/SBML/osx/bundles/061-4841.20080714.bgy8O/iPod1,1_1.1.5_4B1_Restore.ipsw ProtectedAsset/iPod/osx/bundles/061-4957.20080711.Fr43E/iPod1,1_2.0_5A347_Restore.ipsw ProtectedAsset/iPod/osx/bundles/061-4986.20080729.VftgP!/iPod1,1_2.0.1_5B108_Restore.ipsw ProtectedAsset/iPod/osx/bundles/061-5243.20080818.039Ym/iPod1,1_2.0.2_5C1_Restore.ipsw ProtectedAsset/iPod/061-5471.20080909.Plot5/iPod1,1_2.1_5F137_Restore.ipsw ProtectedAsset/iPod/061-5356.20081120.Pt3ct/iPod1,1_2.2_5G77_Restore.ipsw ProtectedAsset/iPod/061-5862.2009127.f2dke/iPod1,1_2.2.1_5H11_Restore.ipsw ProtectedAsset/iPodTouch/061-6579.20090527.Xde3T/iPod1,1_3.0_7A341_Restore.ipsw ProtectedAsset/iPodTouch/061-7161.20090909.VbuyJ/iPod1,1_3.1.1_7C145_Restore.ipsw ProtectedAsset/iPodTouch/061-7266.20091008.SdLPn/iPod1,1_3.1.2_7D11_Restore.ipsw ProtectedAsset/iPodTouch/061-7479.20100202.Secrc/iPod1,1_3.1.3_7E18_Restore.ipsw
iPod Touch (2nd generation) iPod/SBML/osx/bundles/061-5494.20080909.8i9o0/iPod2,1_2.1.1_5F138_Restore.ipsw iPod/SBML/osx/061-5358.20081120.Gtghy/iPod2,1_2.2_5G77a_Restore.ipsw iPhone/061-5863.20090127.rt56K/iPod2,1_2.2.1_5H11a_Restore.ipsw ProtectedAsset/iPodTouch/061-6581.20090527.PiKu7/iPod2,1_3.0_7A341_Restore.ipsw ProtectedAsset/iPodTouch/061-7162.20090909.NkUy7/iPod2,1_3.1.1_7C145_Restore.ipsw ProtectedAsset/iPodTouch/061-7269.20091008.Pfd32/iPod2,1_3.1.2_7D11_Restore.ipsw ProtectedAsset/iPodTouch/061-7471.20100202.4itfh/iPod2,1_3.1.3_7E18_Restore.ipsw iPhone4/061-7435.20100621.tr49t/iPod2,1_4.0_8A293_Restore.ipsw iPhone4/061-8551.20100811.Xcder/iPod2,1_4.0.2_8A400_Restore.ipsw iPhone4/061-7937.20100908.ghj4f/iPod2,1_4.1_8B117_Restore.ipsw
iPod Touch (3rd generation) iPod/SBML/osx/bundles/061-7163.20090909.NtstR/iPod3,1_3.1.1_7C145_Restore.ipsw iPhone/061-7238.20090918.23GhT/iPod3,1_3.1.1_7C146_Restore.ipsw iPhone/061-7271.20091008.Tch23/iPod3,1_3.1.2_7D11_Restore.ipsw iPhone/061-7473.20100202.4i44t/iPod3,1_3.1.3_7E18_Restore.ipsw iPhone4/061-7381.20100621.AzSP9/iPod3,1_4.0_8A293_Restore.ipsw iPhone4/061-8554.20100811.Bgt54/iPod3,1_4.0.2_8A400_Restore.ipsw iPhone4/061-7941.20100908.sV9KE/iPod3,1_4.1_8B117_Restore.ipsw
iPod Touch (4th generation) iPhone4/061-8490.20100901.hyjtR/iPod4,1_4.1_8B117_Restore.ipsw
iPhone iPhone/061-3538.20070629.B7vXa/iPhone1,1_1.0_1A543a_Restore.ipsw iPhone/061-3614.20070731.Nt6Y7/iPhone1,1_1.0.1_1C25_Restore.ipsw iPhone/061-3823.20070821.vormd/iPhone1,1_1.0.2_1C28_Restore.ipsw iPhone/061-3883.20070927.In76t/iPhone1,1_1.1.1_3A109a_Restore.ipsw iPhone/061-4037.20071107.5Bghn/iPhone1,1_1.1.2_3B48b_Restore.ipsw iPhone/061-4061.20080115.4Fvn7/iPhone1,1_1.1.3_4A93_Restore.ipsw iPhone/061-4313.20080226.Sw39i/iPhone1,1_1.1.4_4A102_Restore.ipsw iPhone/061-4956.20080710.V50OI/iPhone1,1_2.0_5A347_Restore.ipsw iPhone/061-5135.20080729.Vfgtr/iPhone1,1_2.0.1_5B108_Restore.ipsw iPhone/061-5246.20080818.2V0hO/iPhone1,1_2.0.2_5C1_Restore.ipsw iPhone/061-5202.20080909.gkbEj/iPhone1,1_2.1_5F136_Restore.ipsw iPhone/061-5779.20081120.Pt5yH/iPhone1,1_2.2_5G77_Restore.ipsw iPhone/061-5830.20090127.Mmni6/iPhone1,1_2.2.1_5H11_Restore.ipsw iPhone/061-6580.20090617.XsP76/iPhone1,1_3.0_7A341_Restore.ipsw iPhone/061-6974.20090731.Cf4Tg/iPhone1,1_3.0.1_7A400_Restore.ipsw iPhone/061-6605.20090909.PQ3ws/iPhone1,1_3.1_7C144_Restore.ipsw iPhone/061-7268.20091008.32pNe/iPhone1,1_3.1.2_7D11_Restore.ipsw iPhone/061-7481.20100202.4orot/iPhone1,1_3.1.3_7E18_Restore.ipsw
iPhone 3G iPhone/061-4955.20080710.bgt53/iPhone1,2_2.0_5A347_Restore.ipsw iPhone/061-5134.20080729.Q2W3E/iPhone1,2_2.0.1_5B108_Restore.ipsw iPhone/061-5241.20080818.t5Fv3/iPhone1,2_2.0.2_5C1_Restore.ipsw iPhone/061-5198.20080909.K3294/iPhone1,2_2.1_5F136_Restore.ipsw iPhone/061-5778.20081120.Aqw4R/iPhone1,2_2.2_5G77_Restore.ipsw iPhone/061-5828.20090127.aQLi8/iPhone1,2_2.2.1_5H11_Restore.ipsw iPhone/061-6578.20090617.VfgtU/iPhone1,2_3.0_7A341_Restore.ipsw iPhone/061-6972.20090731.Zx3Rr/iPhone1,2_3.0.1_7A400_Restore.ipsw iPhone/061-6600.20090909.AwndZ/iPhone1,2_3.1_7C144_Restore.ipsw iPhone/061-7265.20091008.Xsd32/iPhone1,2_3.1.2_7D11_Restore.ipsw iPhone/061-7468.20100202.pbnrt/iPhone1,2_3.1.3_7E18_Restore.ipsw iPhone4/061-7436.20100621.58Yt4/iPhone1,2_4.0_8A293_Restore.ipsw iPhone4/061-8616.20100715.phnt4/iPhone1,2_4.0.1_8A306_Restore.ipsw iPhone4/061-8802.20100811.XcfpR/iPhone1,2_4.0.2_8A400_Restore.ipsw iPhone4/061-7932.20100908.3fgt5/iPhone1,2_4.1_8B117_Restore.ipsw
iPhone 3GS iPhone/061-6582.20090617.LlI87/iPhone2,1_3.0_7A341_Restore.ipsw iPhone/061-6976.20090731.Vgbt5/iPhone2,1_3.0.1_7A400_Restore.ipsw iPhone/061-6609.20090909.mwws4/iPhone2,1_3.1_7C144_Restore.ipsw iPhone/061-7270.20091008.phn32/iPhone2,1_3.1.2_7D11_Restore.ipsw iPhone/061-7472.20100202.8tugj/iPhone2,1_3.1.3_7E18_Restore.ipsw iPhone4/061-7437.20100621.5urG8/iPhone2,1_4.0_8A293_Restore.ipsw iPhone4/061-8618.20100715.Zapn4/iPhone2,1_4.0.1_8A306_Restore.ipsw iPhone4/061-8805.20100811.Dcr4e/iPhone2,1_4.0.2_8A400_Restore.ipsw iPhone4/061-7938.20100908.F3rCk/iPhone2,1_4.1_8B117_Restore.ipsw
iPhone 4 iPhone4/061-7380.20100621,Vfgb5/iPhone3,1_4.0_8A293_Restore.ipsw iPhone4/061-8619.20100715.4Pnsx/iPhone3,1_4.0.1_8A306_Restore.ipsw iPhone4/061-8807.20100811.3Edre/iPhone3,1_4.0.2_8A400_Restore.ipsw iPhone4/061-7939.20100908.Lcyg3/iPhone3,1_4.1_8B117_Restore.ipsw
iPad iPad/061-7987.20100403.mjiTr/iPad1,1_3.2_7B367_Restore.ipsw iPad/061-8282.20100713.vgtgh/iPad1,1_3.2.1_7B405_Restore.ipsw iPad/061-8801.20100811.CvfR5/iPad1,1_3.2.2_7B500_Restore.ipsw
documentation files for iOS:
ipod: 1.1? 1.1.1? 1.1.2? 1.1.3? iPod/SBML/osx/bundles/061-4354.20080226.Bk439/iPodDocumentation_1.1.4.ipd iPod/SBML/osx/bundles/061-4984.20080714.XsEW6/iPodDocumentation_1.1.5.ipd iPod/SBML/osx/bundles/061-4721.20080711.Vgt6I/iPodDocumentation_2.0.ipd 2.0.1? iPhone/061-5339.20080818.nHlp4/iPodDocumentation_2.0.2.ipd 2.1? 2.2? iPod/SBML/osx/061-5928.20090127.ZaspL/iPodDocumentation_2.2.1.ipd 3.0? 3.1.1? 3.1.2? iPhone/061-7821.20100202.AzPyt/iPodDocumentation_3.1.3.ipd iPhone4/061-8460.20100621.BgR5r/iPodDocumentation_4.0.ipd iPhone4/061-8877.20100811.Fvgrt/iPodDocumentation_4.0.2.ipd iPhone4/061-9059.20100901.AzW3P/iPodDocumentation_4.1.ipd
iphone: 1.0.0? 1.0.1? iPhone/061-3823.20070821.vormd/iPhoneDocumentation_1.0.2.ipd 1.1.1? iPhone/061-3992.20071107.3Fvgd/iPhoneDocumentation_1.1.2.ipd 1.1.3? iPhone/061-4355.20080226.Pt57h/iPhoneDocumentation_1.1.4.ipd iPhone/061-4722.20080710.Vf0pW/iPhoneDocumentation_2.0.ipd 2.0.1? iPhone/061-5340.20080818.VB9iU/iPhoneDocumentation_2.0.2.ipd iPhone/061-5390.20080909.azsw3/iPhoneDocumentation_2.1.ipd 2.2? iPhone/061-5920.20090127.vgrt5/iPhoneDocumentation_2.2.1.ipd iPhone/061-6592.20090612.0O8nr/iPhoneDocumentation_3.0.ipd iPhone/061-6994.20090731.Dci8U/iPhoneDocumentation_3.0.1.ipd iPhone/061-7111.20090909.PTgdK/iPhoneDocumentation_3.1.ipd 3.1.2? iPhone/061-7822.20100202.VfP8u/iPhoneDocumentation_3.1.3.ipd iPhone4/061-8461.20100621.Plhn4/iPhoneDocumentation_4.0.ipd iPhone4/061-8694.20100715.pn4nr/iPhoneDocumentation_4.0.1.ipd iPhone4/061-8878.20100811.VPlku/iPhoneDocumentation_4.0.2.ipd iPhone4/061-9060.20100901.Mr5YT/iPhoneDocumentation_4.1.ipd
ipad: iPad/061-8481.20100713.txd34/iPadDocumentation_3.2.ipd 3.2.1? iPad/061-8879.20100811.AsWe3/iPadDocumentation_4.0.2.ipd (this is 3.2.2 for some reason)