Couldn't say for sure exactly where, but I'm sure they get them from a restaurant supplier. I would wager that your local restaurant supply store has something similar, if not identical. A quick search online for "stemless margarita glass" also turned up several definite contenders.
If you have a World Market near you, that might also be a good place to check out. It probably won't be an exact replica, but I guarantee that they carry something very close.
Note: It looks like you may find what you're looking for more quickly by searching for
"stemless martini glass".