Here are the best sources for 3D models:

Thingiverse

https://www.thingiverse.com

MyMiniFactory

https://www.myminifactory.com

YouMagine

https://www.youmagine.com

GrabCAD

https://grabcad.com

Smithsonian

https://3d.si.edu/explorer/

NIH 3D Print Exchange

https://3dprint.nih.gov/discover

Traceparts

https://www.traceparts.com

STL Hive

http://www.stlhive.com