Originally Posted by AlanKHG

There exist SRAM flat-bar shifters:
http://www.bikesonline.com/index.php...-set-Flat-Bar#
That store is the cheapest one I found in a quick search; I have no idea if it's a good one. See if you can find a single shifter on eBay.

You could also team up a thumbie http://www.paulcomp.com/thumbshifters.html with a 10-speed Shimano or SRAM barend. You'd similarly need to do some searching to find only a rear one.