DecalGirl.com are the worlds leading supplier of skins for all consoles, if you want to change your drab GameBoy Advance, GBA SP and GB Micro then give these guys a look.
http://www.decalgirl.com/graphics/gameboy-boutique.jpg
They have the following # of skins.
GB Micro - 162 skins
GB Advance - 132 skins
GBA SP - 60 skins
http://www.decalgirl.com/catimg/gbm-250.jpg
