2826 has really been impressed with SDP-SI (
http://www.sdp-si.com/). There hasn't been a belt part/system that we wanted that they did not have. If you call them up (instead of placing an order online), they will give a pretty nice discount when you tell them the order is for educational purposes.
I have only procured 100 inches of continuous, double sided belt from them, but I would bet 250 inches would not be uncommon.
Word of general advice: Stick with the Kevlar reinforced urethane belt if possible. You will thank yourself later