If you have a Rockler or Woodcraft store closeby, I would check with them first. If not here are a couple of Internet sources that will be more economical than buying from JET.
JET Belt Number: Carlisle Poly-Rib 240J
Link to site for purchase of JET belts;
Company name: Michigan Industrial Belting
Phone number:1-800-778-1650
http://www.mibelting.com/indbelt.htm
Note: These guys don't have on line ordering. You have to call their customer service line.
Here is another one:
http://www.aarubcorubber.com/