If you're a sci-fi fan, you have to go to Starbase 21 just to browse. They have the most extensive inventory of everything Star Trek, Star Wars, etc. Costumes, models, phasers, books, cards... you name it.
They also put on a "Trek Expo" every summer that's a hoot to go to. A group of people come dressed as Klingon's every year, and try to act the part. And you have your people dressed as Vulcans, Ferangi, and everything else. It's fun to go to if you're a fan. Probably even funnier if you're not... you can laugh at all us geeks.