I'd suggest removing the starter motor (if you haven't done already) and then look for part numbers / manufacturer details for the starter motor itself (rather than the bike it's fitted to). You can then search for the brushes for that model of starter.
You may also find that with a little bit of every fine emery paper, you can clean up the existing brushes. This, combined with some contact cleaner, might kick the starter back into life.
Another thing to check is the springs that hold the brushes in place - stretching these slightly could help as well. Don't stretch them much though - it will cause the brushes to wear out more quickly.
This of course all assumes that brushes do prove to be your problem.
Hairsy's post in 1992 Yamaha SR125 Starting motor brushes was marked as the answer