Paid for 3D Print Models
Gambody – https://www.gambody.com
Loot Studio (Miniatures) – https://www.loot-studios.com/
MyMiniFactory – https://www.myminifactory.com/
Free 3D Print Models
3DMag.org – http://3dmag.org/en/market/
3DPrintKings – https://3dpkings.com/
Pinshape – https://pinshape.com/
Printables – https://www.printables.com/
Thingverse – https://www.thingiverse.com
YouMagine – https://www.youmagine.com
3D Artists and Modellers
FotisMint – https://fotismint.com/
Geoffro / Hex3D – https://www.patreon.com/Geoffro
SidNaique – https://www.sidnaique.com/
Engineering Products and Components
www.mcmaster.com – https://www.mcmaster.com/
(Not a 3D Model site per se, but has 3D objects for many of their products)