Was this like a knurled sleeve with tiny grub screws to secure it? I remember seeing something like this in the "new kit" section in a magazine, but it must be about twenty years ago. I can't recall ever seeing one in a shop or seeing one advertised though. The "Collectibles" forum might be a good place to ask if anyone has one to sell. A reasonable lathe engineer should be able to make one for you quite easily if you can't fine one to buy.