I've seen a few of these change hands on ebay over the year, but they are not too common on the used market. However have you tried looking for one new? I'd start with the factory itself - DWH - as they might have one, and I think I saw a new one for sale at 9daysHK too. A lot of camera systems such as Linhof and Alpa use this part to fit Rollei lenses so there may be one out there in the dealer network as well. I think you are looking for part number 12 782.