lik-sang and gbax are probably the best ones, but any decent electronics store should sell them. Before buying one however, check the polarity off your gp32 - I think it's opposite (center negative) to how they normally connect (center positve). Most adaptors come with polarity changers now, but best make sure before buying, so you don't have to take it back.
Get a 3V one, about 300ma if you don't overclock, or more if you do (too much current doesn't matter, too little does). Only ever use 3V supplies however, and make sure they are regulated.