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.