If you're really having trouble finding a used set, and if you have the cash, you could always buy a boat that comes with a set, then flip the boat and keep the Cat Trax. This takes cash and cajones (you don't want to get stuck with the boat)...

This works best with a boat like a Hobie Wave, you can usually turn them around very quickly. During the start of sailing season, they don't last on CL for very long.

Mike