i've snuck a couple of DS cards in my wallet in a pinch. Better than having them unknowingly fall out of a pocket when you sit down.
Edit: And like someone else said, look into cases for SmartMedia, MultiMedia (MMC), or Secure Digital (SD) cards. They're roughly the same size as DS cards, and as long as they're not fitted, they should work. i saw a zippered Case Logic case that held eight SD cards for something like $5 at WalMart. It was roughly the size of an empty wallet.