I've seen them on ebay once or twice, but I don't look regularly. I bought one there. I suggested Leica simply because I thought they might have some new-old-stock. The last suggestion (a long shot) is to try Kienzle. If you look at their web site, they make bits for the older Leitz enlargers and may be able to help you. Lastly, if you can find a regular one, it would be pretty easy for a local machine shop (or do it yourself with a file) to mill the opening oversize.

Mark