While you can buy a four string plus a separate C, most local dealers will only stock a very small selection. You probably need to stick with the larger on-line string dealers (like juststrings.com) to locate the set you're looking for. It looks like GHS Boomers and La Bella Slappers are both available in a native high-C five string set.
Alternately, I'd contact one of the stronger dealers out there, like Beaver at Bass Central, and see what they can get for you. Beaver has a set listed from R. Cocco on his site and is a pleasure to do business with. He's also been one of the larger Alembic dealers for years and knows our instruments pretty well.
Remember that the type, construction and gauge of the strings will have a significant affect on your tone. You may not have a lot of easy choices, but be prepared to buy a six string set if you have to in order to get the right set for you.
-bob