The following is a curated selection of stores where I bought components for my 3D printers together with a short "buying experience". I hope it helps.
By EU I mean unified market (sorry UK). Suggestions are welcome. Random order.
filafarm.de Custom beds, heaters, printing surfaces.
Good quality but very expensive, also pretty long lead times. I bought a custom alu bed, 8mm thickness, it was pretty straight at the beginning but bent pretty heavily over time. I don't think I would buy again, but the silicone heater and the printing surfaces are very good.
cnc-store.eu linear rails, ballscrews, leadscrews, CNC stuff
I buy original hiwin rails from them. Very good quality and shipping is top notch to avoid bending. Not the cheapest but it's local to me so still worth it.
tuli-shop.com Linear motion
I got ground ballscrews and variour linear rails. They do custom work, very good quality but not cheap.
3djake.com Generic 3d printing store
Good filament selection, bondtech and phaetus reseller. Good as it's easy to reach the free shipping tier.
omc-stepperonline.com Stepper motors
It was okay few years ago, now I would suggest to go LDO instead.
reprapworld.eu Generic 3d printing store
A bit of everything at a very average quality. Price can be good, I bought some components in the past but I don't use them anymore.
makerbeam.com Small aluminum profiles
Only place where I can find 15x15 and 10x10 profiles. Shipping is not the cheapest.
aprintapro.com Bed adhesive
Like VM polymer goo, but cheaper.
christmas3dp.com Various 3d components for printer
Very little selection, but the guy is very passionate and makes nice youtube videos too. They also have bed adhesive based on the PVP + PVOH formula. Shipping was pretty slow.
hardware-cnc.nl CNC stuff mostly
Motion systems, spindles, electronics, mostly for CNC. You may still find some components for 3d printers (ballscrews, couplings, ...)
clever3d.de Custom aluminum bed
Contact them via email, they can do custom work. Not the fastest to reply.
3dprima.com Generic 3d printing store
Good selection of filaments and spare parts. Their flex plates are decent. Also PrimaSelect filaments are not bad.
Conrad Tools, electronics, ...
Sometimes you can find good deals on original meanwell PSU (and various hardware).
lecktor.com Mostly Voron supplier
I bought LDO motors and various components (pulleys, belts, ...). Very fast shipping. Would recommend.
caribou3d.com Generic 3d printing store
Good quality stuff. Shipping is a bit expensive depending on where you are located, but overall a good option.
ratrig.com 3D printers manufacturer
Never bought a ratrig product but they have a lot components, generally good quality. I got some leadscrews that are incredibly straight. Shipping was fast too.
dropeffect.com Extruder
If you need to print elastic filaments this might be a good option
sintratec.com 3D printer manufacturer
This is kinda off-topic but this is the only "low-cost" SLS printer I know of.
metall-jobst.de Steel flexible plates
High quality decent priced custom flexible steel plates. Great service, super fast.
screwsandmore.de Fasteners
Huge selection of screws and fasteners. Good quality fast service.
schrauben-expert.de Fasteners
Never bought from them but it was recommended to me.
schrauben-niro.de Fasteners
Great quality and good price.
motedis.com Generic components for 3d printers
Quality is generally low. Buy extrusions only if you can cut them your own because their cutting precision is very low. Don't buy screws, they are very low quality. Generally I don't recommend.
dold-mechatronik.de Generic components for 3d printers (and more)
Profiles cutting service is decent. I also tried some of the linear rails they have and they are good quality. The 9mm carriage is nice (the one with tight tolerance) but rail is a bit too flexy (be sure to put it on a rigid support). Ballscrews and leadscrews are good but not great imho.
3do.eu 3D printing stuff
Good filaments. They sell mostly Mellow stuff. Good quality, prices are okay. I ordered a belt once and they decided to cut it randomly, which is not nice. Very easy to reach also on Discord.
tme.eu Electronics and Hardware
One of the few places where you can buy shoulder bolts in EU.
mouser.com Electronics and Hardware
Not EU but taxes are paid directly at checkout. I generally buy switches, endstops and fans from them. Fast FedEx shipping.
bondtech.se The extruder guys
Don't buy unless you really have to. Go to any of the many EU vendors, they generally have better pricing and often free shipping.
dhm-online.com Generic 3d printing store
Good selection, average pricing. I keep it on my radar just because it's domestic to me.
keenovo.store Bed heaters
Custom bed heaters. It takes about 30 days for a custom heater, but I've not been hit by import taxes.
Mellow on aliexpress 3D printing stuff
Very well known, generally high quality, but they are a bit hit-and-miss. They solve any problem you may have very quickly though. I've not been hit by import taxes.
accu.co.uk Generic hardware
I bought very good quality shoulder bolts from them. Pretty expensive.
robotdigg.com 3d printers and CNC stuff
Never bought from them but everybody recommend them. It's also the only place where you can buy Delta stuff.
- Don't buy shoulder bolts from random aliexpress vendors. They are terrible, you are better off using M5 screws.
- If you don't have an enclosure and can't print ABS/ASA a good alternative is ABS+ and I generally prefer it to PETG. Be careful though! Most ABS+ are shit! Don't trust low priced ABS+. eSun ABS+ is terrible (they changed formula and the new one has very poor layer adhesion)! The best I've found so far is FormFutura TitanX.
- If I turn off completely part cooling and use a very good bed adhesive I'm able to print ABS and ASA without an enclosure.