A mouse for the C64 is not a common thing. Its a shame as there is actualy a whole mouse based OS (geos) for the C64.
If you cant get a C64 mouse, why not interface a common cheap ps/2 mouse!
HackaDay recently found some Flickr photos of just such a project.
Apparently a project born from the love or art. Pixel Art. How to create great art on an old 8Bit computer? A mouse of course.
I tracked down the project page that seems to have all the docs available including code to build your own!
Great work that needs recognition.