Eagle Belting Company is a major manufacturer of urethane belting.
They have V, round, and flat belting in a variety of sizes.  They
might be able to provide referrals to companies that make custom
belts from their materials.

  Eagle Belting Company
  1683 S. Mt. Prospect Road
  Des Plaines, IL 60018  tel: (847) 297-8200

For those who like web searching, try looking for urethane belting.
There has to be something out there.

Most shops that sell bearings and drive components have access to
urethane belting materials and custom manufacturing.  I checked the
W. W. Grainger catalog and they don't have it.  They are expensive and
difficult to buy from if you do not do business with them regularly.

Spencer Chase
http://nt.mcn.org/spencer/