Saw two at Hershey two weeks ago restored and ready to install for $600. each. Saw one at Carlisle the week before in need of restoration for $225. These things don't appear to be cheap or plentiful. Either bite the bullet or carry a blanket.
Or, you can pick one from this list of vintage automobile heaters that aren't necessarily Ford nor 1939 but would probably work and many a lot cheaper. A heater's a heater. Right?