Sorry to hear about the pinch roller...check out this thread from 2009 about finding a replacement. Maybe one of the links there will lead to something. Have you tried calling Tascam directly? That's how I got my belt. I've never had to change the roller. Cleaning mine turned out to be good enough - at least for now>
http://homerecording.com/bbs/general-discussions/analog-only/rubber-wheel-tascam-688-a-279681/2/
http://homerecording.com/bbs/general-discussions/analog-only/rubber-wheel-tascam-688-a-279681/2/