Some links I use from time to time. This helps me remember them when I need, and maybe this is useful for you as well!
3D Models for 3D Printing
Probably the most known place to find 3D models to 3D print.
Second most used site for me to find 3D models. It has less models, and quite a few are not free. Still a good go-to option.
I use this from time to time. Quite interesting one, with potential to turn into the #1 in the future.