Custom DS cases are indeed very cool, but if you're wanting something comparable to the old Gameboy cart plastic cases, look around the net for GBA plastic cases, they're just about the same size as the old GB cases, but have slight indentions on both ends to store 2 GBA carts inside each clear case. Usually can be found in packs of 3-4. Or simply do a search on eBay for something like "GBA case" to see what you like the best, there's been plenty of different storage options made over the years, but you won't find them in retail anymore.