Superior Beverages of Phoenixville is the distributor in your area. I don't know all the flavors they carry, but Genuardi's usually has them.
Davis Beverages is the next closest.. I know they have the Parkesburg area.. but I am not 100% sure of where their territory starts. They have a pretty good selection in the super wally there, and I know they recently started buying Diet Rite 12pks from RC Winchester (I saw them in Lancaster).
If you ever travelling to Baltimore, you will be able to find them down that way. Canada Dry Potomac makes them in 20oz 6pk, and now 12pk as well. I'm not sure if they are making the cans or buying from Winchester (I didn't get a good look at the package).
Mars Supermarkets usually carries all the flavors -- they have quite a few locations around Baltimore.