El Pescado
Member
If you guys are just after utility and not a collector's item, Hori has a case that can hold 24 carts: http://www.play-asia.com/3DS_Card_Case_24_Clear/paOS-13-71-18g-49-en-70-43a8.html
There's also a guy on a Nintendo fan forum I visit who has 8 of the Club Nintendo card cases for sale around $25 each - I wonder how he accumulated enough coins for all of that.
My friend has that Hori case and swears that it's complete garbage and split some of his games open just for the record.