Andrew R Stewart
Join Date: Feb 2012
Location: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Raleigh Pro, Trek Cycle Cross, Mongoose tomac ATB, GT Bravado ATB, JCP Folder
There are many (as in dozen+) ders that could work. First filter is the seat tube diameter. Second is whether the der is intended (speced for) a triple crank. Third is the cable routing (from below or above, you want below route). Then the cage shape, this spec can be hard to determine without mounting the der but you want the inner cage plate to clear the middle ring's teeth when the outer plate's height off the large ring is correct. Next is the outer plate's contour/thickness and the ring to arm space, when finally adjusted the outer plate needs to clear the arm's inside surface.
So many ders will fit the bill. Surely there's a LBS in town that you can start with for a used one from their recycle box? If you try the Ebay route then you need to bone up on the above factors before you take the plunge and bid on one. Andy.