Nintendo 3DS Game Card Case
You can find the Hori 3DS game card case for sale here. It has a lot of good feedback and according to those who bought the item, it's small, durable, it can store up to 24 game cards and it has a one slot (holder) for an SD Card (in case you need a backup). Did I mention that it can also hold your old DS games?
The game card case has been officially licensed by Nintendo and comes in Black, Blue and Clear colors for only US$ 11.99.