Give Antique tyres a call. They are a little expensive and have poor customer service but they cater for a limited market.
After a few years of calling them, asking them if they can get 1200x20 bar treads and being told they couldn't, one fellow said "This is the manufacturer we deal with. We can only get those."
When I pointed out to him that manufacturer made 1200x20 bar treads, he was very surprised and ordered some for me after three further phone calls.