you're looking for a protective shell or something? I don't think there're any shells that have cart storage built in
I haven't seen one either. The closest thing is a grip with a cartridge holder underneath, which is fine at home but not really convenient for travel.
@OP I would suggest a pouch. It's a good idea anyway to protect your shiny new device, and you get some room for a USB charging cable, screen cloth, mSD reader, and a couple of cartridges. All useful things to have with you that don't take up much space so they will fit in a pouch
Any regular 3DS XL pouch should fit, there aren't many n3DS specific accessories yet.