Hi Group,  A large well-established hardware store (not sure about
the new "big box" stores) will carry 1/8-inch "O" rings in various
sizes.  I got one that fit at Andrews Hardware in Los Angeles, which
no longer is in business.  Some microwave oven turntable belts will
also work.

I have solved the problem on my Pianolodeon with a 16 mm motion picture
projector spring wire drive belt.  Yes, I do collect other old and
strange things.  The belts are adjustable in size and do not stretch
over time.  At least not in 10 years.  Check the yellow pages for
audio-visual equipment repair firms.

Jack M. Conway, Los Angeles, California, USA