http://www.spriproducts.com/Item.aspx?ItemID=152
The link is to the manual for it, but you get the point.
I got my tube for $12.Hint: I got the purple one for "extremely fit" because I first bought the red (moderately fit) one and opened it up and thought it was really wimpy.
So be cautious... however, I have been working out quite a bit for several months.It comes with a door strap and sets of instructions for exercise.

Oh well.... 
