Head over heels for a guy I met briefly the other day... What should I do? Should I contact him?

Well, it sounds like you really do like him. The only way to find out how he feels is by speaking to him again. Maybe you should add him on facebook, and send him a little message? I mean, like you said, it's not everyday you go to a costume shop. So going in there again when he's there might be a little awkward. Plus if he doesn't reply on facebook or doesn't seem interested, you're not going to be going back to the costume shop anytime soon. Maybe suggest him going to the convention? Just see how things go. Good luck! :)