Buy things at GameStop? Next time you buy something there, casually mention that you bought a game (Kameo) at another GameStop but they didn't have the case or manual, just the disc. Ask if they could spare the case/manual from one of their copies, since it kinda bothers you (and any GS employee knows that most people could give two shits about the proper cases).
I'd have no problem with that if someone came in while I was working and I had seen the guy at least once before.
Other than that, just keep your eyes on eBay, or hope someone here has one.