I am not sure where you could find any.. but I would recommend the avid ...hmm what was it called.. the tri-align? I have them on two of my tandems. They are very adjustable and quite powerful. They do not need a travel agent.. or anything like that. I personally prefer them to any v brake I have used..and I even like them better than the magura's
I have them on a road tandem with road levers... and a moutain tandem with their mountain levers.
glenn