I see them quite regularly on Ebay - from memory they are about $50-60 but I don't know what they end for.

I know that from reading here there are some alternatives but people haven't been as happy with them.

There are also instructions somewhere on the web (but I have lost the link) that tells you everything you need to buy at a hardware shop to install a DIY one - but personally I would probably buy the real think and not muck about for a day trying to find all the right pieces and them put them in the right order.