There's a device pictured here:
http://www.heikes-heels.de/english/t...training01.htm
If you want something that will do sort of the same thing but not be so expensive or so extreme, there's this. It's called the Pro-Arch. www.discountdance.com, click on 'gifts and accessories', page 3.
(Tried to post a link, but it didn't work.)