if your talking about a gp32 (the white one with two buttons) they are out of production and ebay is the only way to go. thats how i got mine
if you mean the gp2x (black, four buttons) a very good source is
http://www.gp2x.co.uk/ while not in the states it is a very good site.
if you want to walk into a brick and mortar store and buy one, sorry but you cant, online only