Alan, your options are almost endless. Deere offered them, as did Cozy Cab and I am sure the other cab makers. After that route, you will also be able to find generic ones through farm supply stores and beyond that, you can look in the auto wreckers for a rear heater out of an older suburban or some extended full size vans.
Keep in mind, bigger isn't better in this case. Space inside the cab is limited and you'll find that very little heat is required to make it warm.