those are interuptor levers designed for cyclocross. paul makes expensive ones, and pyramid and tektro make cheap ones. lbs or google "cyclocross levers." they are designed for road bars, or both the fat and skinny kind.

in my humble (probably minority) opinion, those things suck as a primary brake on a fixed gear. when going fast, you are probably not on the flats. when you are on the flats, you aren't going fast enough to need the brake. go for a set of road levers. if you take it to the track, just swap the bars and pull the brake.