About a month ago, Nintendo of Japan added to its Club Nintendo reward stash something called the Nintendo 3DS Card Case 18.
Admittedly, the item's name isn't the most creative--although it is descriptive. (As you've probably already guessed, the Nintendo 3DS Card Case 18 is an accessory that holds up to 18 DS and/or 3DS cards.)
I don't know this for sure, but I have a feeling the case's name is lacking in creativity because its designers focused all of their attention on its awesome alternate covers.
You see, although the case's standard cover, seen in the photo above, is a bit bland, its alternate covers (each "order" comes with three of them) are anything but. For example, here's one that includes sprites from The Legend of Zelda:
Additional covers--featuring Bowser, Mario, Pikmin's Olimar and more--can be seen in this Flickr photo set.
These cases are now available via the North American Club Nintendo "shop," too, by the way. They're called "Nintendo 3DS Game Card Cases" (I guess the "18" part was too exciting for them) in this region, but the alternate covers appear to be the same.