There are road bike shifters that are designed for flat (or trekking) bars.
http://www.amazon.com/Shimano-SL-R44.../dp/B001ON4XI2. You will need brake levers designed for cantilever brakes (not v-brakes):
http://www.amazon.com/Shimano-BL-R55...d_sim_sbs_sg_1. I'm pretty sure if you have a double crank that those shifters will work, you just don't use the 3rd click. If I'm wrong, I'm sure someone will clarify. Good luck.