I bought something similiar at either Walmart or Meijer for about $15. It is in the exercise equipment section. Like someone else posted, it is a core building tool. I used it when I first got it and boy did it really make my lats and abdomen ache after awhile but then again, it was doing its job! I must have taken it off my chair for some reason and now I have no idea where it is! It definately made me sit up straighter and I always felt my caboose was tilting off the edge. (Which is probably the purpose of it!)
Here is a link to one similiar to the one I purchased
http://www.walmart.com/ip/Altus-Core...-Disc/10270007