I made a couple of shaft extenders that look just like that. I started with a piece of brass tube, tapped the ID and used red Loctite to secure a short length of threaded rod about half in and half out of the tube. I think I got all the parts I needed at McMaster Carr. I'd have to check to see if I have part numbers if you're interested.
I knew about the full longer replacement shafts but I've never seen extenders like these available before. I wanted to be able to change stick lengths on the fly.
Another option would be to make your own extender with magnetic attachments similar to how Le Chuck made his removable flight stick in this thread:
http://forum.arcadecontrols.com/index.php?topic=116602.0ETA: I made mine for JLW sticks. Not sure if JLFs would be the same size.