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