Spikethebest has it, or you can buy it at most "ethnic" grocery stores and frequently at regular grocery stores. Its also called spineless opuntia cactus. What Cory(spikethebest) has will be fresher and better and he also has the much sought after fruits.
I think I have one of the only red foots that will NOT touch prickly pear cactus.
Well, I guess I shouldn't say that. He wouldn't touch it when he was younger (not too well, anyway), but he's turned into an eating machine so he might take it now.
Many feeder supply places (mouse feeders, crickets, rats, worms, etc...) here in Alberta and over in British Columbia also sell prickly pear, fresh picked, and bagged whole. Fairly good price for the quantity you get, but I'm waiting for another reptile show to pick some up myself.