Probably the main goto site for 3D printable stuff, but it all depends on what you're looking for.Btw, is it thingiverse which is the goto for print files, or are there better sites?
Thingiverse isn't the easiest to find stuff on, but it does contain a lot of models.
If you're looking for something more engineering related, there's nothing stopping you from using sites like GrabCad.
All you need is a suitable 3D model, ideally in STL format.