There are many online libraries (repositories) from where you can choose the 3D models. The files contained in these sites are ready to be printed and sometimes you can even customize the chosen object through simple interfaces. Each user can upload their own files and make them available contributing to the growth of the library.
Among the most widely used repository:
3D Stuff Share:
In particular, we recommend YouMagine and Thingiverse for the quality of the files and the rich choice.