There are some sources where you can download free STL files of 3D designs for 3D printing and some are paid designs, below are the sources you can visit those for readily available 3D models in STL format.

Thingiverse

Pinshape

YouMagine

My Mini Factory

GrabCAD

Autodesk 123D

Repables

ShapeDo

Posted in: 3D Designs