There's two Metroid options, including the one pictured above and a Samus vs. metroid case. If you prefer the Legend of Zelda as your game of choice, though, she also has a case featuring Link and another with a sparkly Triforce.
The cutest of all, though, are the Mario-themed cart cases, which are shaped like little boxes and can hold about nine or ten games. The game holders are available in red mushroom, green mushroom, or question block designs.
Currently they're not too expensive for handmade items ($18 including shipping and handling for the DS Lite cases, $11 for the game cases), but of course, you never know with eBay.