GrabCAD

http://grabcad.com/home

In searching for sites that contained a large number of STL files, I stumbled across GrabCAD. This site contains over 80,000 STL files. (Most of the files on this site are engineering based, but there exists a handful of mathematical objects such as knots, fractals, and klein bottles). It also has a program that may come in useful at some point called GrabCAD. GrabCAD theoretically is beneficial for getting external input on your 3D model. However, upon further inspection, it may be no more effective than plugging a 3D file into a shared Google Drive folder.

All the same, go check it out! You might find something awesome.

Thingiverse

http://www.thingiverse.com/

Like GrabCAD (http://grabcad.com/), This site allows you to download STLs from a large selection of user-made files. Unlike GrabCAD, you do not need to make an account to download the 'thing's. There seems to be quite a large selection of mathematical type of objects contained within the site. Beyond that, one is able to find a large assortment of 3D objects from jewelry to gadgets. One of the things interesting I found on here was the ability to edit some of the projects and the capabilities these projects had. For example, there was a project I found that was able to take any image you wanted to make 3D and place it within a 100x100 mm square frame. (http://www.thingiverse.com/thing:74322). Altogether, this site is a good go-to for quickly finding a variety of 3D printable objects.