I think you can get them as Herbert Capstan lathe toolholders. I don’t have any spare, but have never had any trouble getting them.Toolmex do them I think. DHS got my last one for me
With a 1 Mt reamer its not an overly difficult thing to make anyway. Buy a length of 1″PGMS, set it up in a 4 jaw, Rough drill, and then bore. Finish ream and you should have one pretty smartish!