Poly – Browse, Discover and Download 3D Objects
There are lots of sites full of 3D models.- Thingiverse- 3D Content Central.  http://www.3dcontentcentral.com- GrabCAD. https://grabcad.com/library- 3D Part Library.  http://www.3dpartlib.com/- 3D Warehouse.  https://3dwarehouse.sketchup.comThe ones for CAD are more useful, because they usually have the dimensions right. You download those because you want the parts to fit. For competitive reasons, they tend to be good looking models, even though that's not strictly necessary.