The real issue is finding a flatwound that is offered in a 6-string set, since that is about the only way that a single C might be available. I have used the TI Jazz Flats like this and they are very good E-C. I have been directed to check out Elites, but I never found a source for the high-C. Both of my fretless 5's are strung EADGC and I use Zon Nickels on both of them. Before finding a source for singles I just bought 6 string sets and ate the cost of the B. In many cases it is either the same $$$, or even cheaper since there is a premium placed on singles by many sellers.
Check out something like:
http://www.google.com/search?hl=en&q...=Google+Search
Pyramids, NS, Rotos, and TIs jump right out.