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.

Just another option. Not much to say.

GrabCad Community
One more option. Again not much to say.

Cults 3D
And one more option.

One more option to the list.