By far the best site to find free 3D models is Thingiverse.
I have a small number of my creations on that site.
For toothed gear wheels, this is a possible source for 3D models.
For example, this Parametric Pully object looks perfect for playing with toothed wheel sizes! It has a .scad source file, so just varying a number or two lets you change the design for printing.
As noted elsewhere, see OpenSCAD for the 3D software.
It's a matter of conjecture at this point if 3D printed pulleys will hold up to the rigour of rotating a disk at 750rpm, but I'm going to try!