r/MechanicalKeyboards - for inspiration, resources, and to ask questions
Australian Mechanical Keyboards Facebook Group
I have a blog post covering my first build experience
Here's a great YouTube channel with lots of keyboard build videos including this great introductory video
Soldering introduction video (plus plenty more if you search YouTube)
KBDFans and KPRepublic - popular Chinese stores for keyboard parts
AliExpress also has storefronts for the above and various other keyboard sellers if you search for parts
Massdrop host time-limited sales for all sorts of keyboard kits/accessories, though you need to sign up view
Keeb.io is an American seller that has lots of split keyboard designs