Awhile back 8-9 months there was a guy in New York collecting these and were selling parts, but I lost his email and can't remember his name.
A customer bought brand new belts and manuals from him.
You can visit Yahoo's Forum for Reel to Reel and ask those guys. http://groups.yahoo.com/group/reeltoreel/
A rubber band of appropriate length & width will suffice as a palliative. However it might not last as long as an original and might shed particles; it might ride up on the rim of the drive shaft giving lower speed. If short, it might also give excessive friction on bearings.