In my experience cross levers are often sold at either 26.0mm or 31.8mm with an included 24.4 shim, the second option looks kind of jankey when shimmed in my opinion but it's affordable and versatile if you switch bars later since 31.8 can be shimmed down to pretty much anything.
Personally I'd measure and get the right one, though, it just looks cleaner. Use a (metric) ruler and measure how wide your bar is at the point you want to mount the brake. 26.0mm seems to be less common except in cases where the part the stem is mounted to extends out and you want to mount it as close to the center of your bars as possible (I have to use one to mount to the center of my Nitto RB-021s).
I guess, what bars do you have? Can you post a photograph? Do you know your stem clamp diameter? It's often pretty easy to guess your bar diameter from that if you don't have the ability to measure it.
Anyway Tektro, Origin8, and Soma make affordable ones in every size (that clamp on so you don't need to rewrap your bars to put it on), I bought a Paul one because I liked it and had some extra cash at the time.
Last edited by nick1click; 11-26-12 at 02:19 PM.
Reason: forgot how to speak english for a second there