BreakmanX.com

I don't know if they're available in the US or not, but these things are pretty nifty. They come in grey, blue, and black. The cartridge holder can hold 2 games and the connecter cover even flips open when you need to use it. I'm sure I'll pick these up once I get an SP.

http://www.play-asia.com/paOS-17-74-c3.html