I got a blue "Mesh Pouch Micro" from Play-Asia. I think it's great. It's a bit on the bulky side but it's really light, so it doesn't feel annoying in your pocket. You could throw your gameboy at a wall or person with it, and it would be okay.
Otherwise, I saw GBM "Sox" at Gamestop for $4.99. They seem perfect if you prefer small case size over protection.