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I've never purchased from them myself but Arizona Aquatic Gardens advertises that they have SAEs (see http://www.azgardens.com/algae_eating_fish.php). My LFS has SAEs on a regular basis but calls them Thai Flying Foxes.
Most of the decent LFSs around here have them, but as others have mentioned they are usually mislabeled. The ones I have seen at the LFS are usually very small (about 1"), thin little guys, but they fill out and grow fast. I think their availability is somewhat seasonal as well. Be sure you can identify the real ones from the false....maybe bring a pic to your LFS and ask them to order some for you?