You probably won't be able to find an off-the-shelf finished belt of
the right size.  I suspect that you'll have to buy the material and
clip kits or have a belting company make up what you need.  Shouldn't
be a big deal for them.  McMaster-Carr does have some premade belts.

  http://www.mcmaster.com/param/asp/desc.asp?desc=Flat%20Belts
  http://www.mcmaster.com/param/asp/desc.asp?desc=Flat%20Belt%20Pulleys

Since I was looking, I found a few other interesting pages:

  http://engines.rustyiron.com/rig/belting.html
  http://www.pagebelting.com/belting.html
  http://www.assemblymag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,6493,110490,00.html
  http://www.owwm.com/FAQ/Belting.asp

z!
Carl Zwanzig