Never seen these extenders before and here are 2 on UK ebay at moment.
http://tinyurl.com/3mtgap
Sorry the link will take you to page 3 for some reason,. But once you are there you can look round.
I am sure I have seen aftermarket ones of these advertised recently, probably on ebay. I've had a look but cant see them there now. But these things come and go frequently.
