Not sure how big your machine is, but check out a skid steer (Bobcat, John Deere, Cat) dealership. Skid steers can be equipped with metal or rubber tracks, some of which simply mount over the machine's tires.
New sets can be pricey, but the dealer may have an old, "worn out" set lying around, or be able to direct you toward a customer that just replaced them. The tracks may be too worn to function properly on a money- making piece of equipment, but I imagine that they would have plenty of life left for a hobby/ project application.