Lets compare sources and get a list going. Here is mine so far, give or take coffee. Add to the list for everyone.
https://www.yobi3d.com/
https://www.cgtrader.com/
https://www.youmagine.com/
https://3d.si.edu/
https://grabcad.com/library
https://www.myminifactory.com/
https://cults3d.com/en
https://pinshape.com/
https://www.tinkercad.com/things/
http://www.yeggi.com/
https://www.thingiverse.com/
https://www.threeding.com/
https://www.yobi3d.com/
https://www.cgtrader.com/
https://www.youmagine.com/
https://3d.si.edu/
https://grabcad.com/library
https://www.myminifactory.com/
https://cults3d.com/en
https://pinshape.com/
https://www.tinkercad.com/things/
http://www.yeggi.com/
https://www.thingiverse.com/
https://www.threeding.com/