A couple places I think are worth visiting.
My Mini Factory – Science Fiction, Cyberpunk, Fantasy and Horror print models.
Cyberpunk and SciFi
Patreon – Unit-9
Patreon – Papsikels
Patreon – Cyber-Forge
Patreon – Print Minis
MMF – Project Mobius Tribe
MMF – Loot Games Tribe
Fantasy
ArchVillain Games
Bestiarum
Lord of the Print
Mini Monster Mayhem
Mammoth Factory
General models assorted genres.
Extra Guy
Thingiverse – Anything you can think of free stuff,
Black Site Studios – Miniatures and scenic items for table top gaming
Gambody – High quality, large scale models
Printing Materials and Tools
3D Print Monkey – Cheapest option in my part of the world for high quality resins and other materials.
Scaling
Online Scale Calculator – 3D Model Scaling calculator – for converting different scales to the one you want.
Wargaming 3D – Scale Conversion for 3D Printing,