If you can find the Rainbow Six companion disc, it comes in a regular Xbox case, some places pennied it out. Demos and things of that sort are good sources of new cases. I don't think they sell actual Xbox cases by themselves, but there are look alikes out there.
Red Octane has a 2.14 lifetime rating at resellerratings and charges $8 shipping. I wanted a game manual from them and they wanted $8 to ship. $8 when it would probably cost them 37 cents.