I use Thera Band rubber, made by the Hygenic corp., you can find it on ebay.
The flat band type delivers the best performance, as you can cut it in a tapered fashion (so it is much wider at the fork than at the pouch). This way, the "payload" is much lower, more projectile speed at a lower draw.
You need a metal ruler and a roll cutter to cut the 14cm wide rubber into the desired dimensions.
Jörg