http://www.prowriststraps.com/
http://www.andersonpowerlifting.com/

There is only one slingshot... made by the powerlifter that sells them If you want a cheap version, just make a version from this:
http://www.ojcommerce.com/img/prods/med/fab105213_cando_low_powder_exercise_bands.jpg

Thats just what it is... I've personally only seen them at a good exercise store that sold the stuff by the foot. I'm guessing thats what the origional slingshot was made from.

Strengthshop in the uk do their own version of a slingshot, as do Metal.

IPF approved brands are Titan, Inzer, Metal for most equipment