+1 Mountain bike brake levers often have the wrong brake lever ratio (I almost made that mistake, someone on the list bailed me out). You can buy the right kind of lever, but check with your local shop. Down tube shift levers are the easiest option, you can also buy thumb versions. I took a set off of a Trek 330 a while back, converting it back to drop bars. Reused the parts on another project unfortunately. Also you could use old dia compe brake levers, like you see on Schwinn Suburbans and similar. Find someone with an ample parts bin, and they can probably pull out just what you need.

This is yet another good case for finding a donor bike. I picked up a Schwinn Suburban last week for $8, has the bars and brake levers you would need. I will end up rebuilding the Suburban, as it is actually in good shape.