Try the skijoring supply houses too. There are some pretty nice harnesses out there that produce harnesses of various sizes. The harnesses I have could easily double for roaring harnesses. Mine were privately made many years ago before the sport became somewhat mainstream but there are many companies producing harnesses today.
Skijoring is basically dog sledding on cross country skis. The dog does not provide all the locomotive power but is used as an assist. If one gets more than one dog to pull, hang on! I've had three dogs pulling at once and one can really get moving - on the skis or not. I ran multiple dogs in a fan set rather than inline as it fit better with our abilities and objectives.
It is great winter excersize for both dog and handler and is a whole lot of fun. I've had dogs from 30 pounds to nearly 100 pounds in harness and all enjoyed it.
Have you seen this done with the big tired, adult scooters (urban mushing)? That looks fun and like something that'd give a dog a good workout.
