I see them on eBay all the time. I would just buy one and tell them to put it in. Unless a shop specializes in classic vehicles, they won't have a book with listings of anything that is no longer made. Just because they can't identify 50 year old parts doesn't mean that they don't know what they are doing. You must remember that a lot of shops charge you for what you don't know. If you go to someone who has the books on these old transmission, you will probably pay for what they know. If these guys know transmissions, you will probably get a better deal from them. The transmission you have is probably one of the simplest ever built.