I will start with Printables.com, the Prusa created site for sharing models. I was early to join their beta project and have seen them grow into a robust platform. I am finding their contest inspiring and appreciate their intent to keep free models free while working to create an environment designers can flourish.
Thingiverse.com is from UltiMaker and is one of the longest standing platforms. There was a period of time that the Thingiverse platform was not stable, which is when I switched to using Printables.com. I visited recently and they appear to be stable now so I can recommend them as another good source.
MakerWorld.com is from Bambu Labs and seems to be a strong contender. Bambu is newer with an emphasis on multiple color printing. There are a lot of good colored models and their selection is rapidly growing.
Summary
I have missed a lot more than I have covered but hopefully this will help you start finding models to print.