Why not make your own?
It should be fairly simple to press your GP2X into some elastomer... either polymer clay or silly putty.
Then just slice off the clay, reassemble, and make a plug. Then you can make all the molds you want, and just get some methyl-methacrylate resin and make a nice hard plastic case.
That's what i'm going to do when I have scads of free time in a week or two.
LED's inside and all.
Maybe i'll use this styrene resin for fiberglass instead of the methacrylate. That'll give me a brownish color, or epoxy resin would be clear-ish and I just add some food dye. Oil based dyes work well with polymers, given their non-water-soluble nature.
White, however... would require a white plastic material. Why not just take your existing case apart and spray it?