Aw, that bites that most of the stores near you don't have many games. It would definitely be the easiest way to go!
Hopefully, as you stated in your original post, some members on the forums will have put their Genesis games into UGC's, and not need the original clamshells. That totally makes sense, and you're right, someone has to have a stash of them lying around somewhere.
Aside from that, you may have luck winning an online auction for a lot of 20 or 30 games. I do a lot of buying and selling on a "certain major online auction site," and I can vouch that there are some sellers who have a pile of outdated sports games / overstock copies of the "Sports Talk" series of games that sell for next-to-nothing. Sega's Sports Talk series of sports games came in clamshell cases with the official Sega logo inside, hang tags on the back, and the cartridges were the regular size, as well. If you have to go the online route, that would probably be the best way to go to get official Sega cases.
I hope someone here can help you! How many cases are you looking for? I may be able to scrounge some up from boxes in my closet, though I think I'd only have five or six at the most. Would that help?