I had bought the DynaDisc by Exertools [it is the same thing as the Gypsy product only it was much cheaper when I bought mine] years ago to use on my old sewing chair ... and still take it to sew-in's and such. I think you could find something similar in a sports store or sports/fitness website. I like mine because I cna change t he firmness by adjusting how much air is in it.
That thing that turns is awfully hard to sit on for long as far as I am concerned. It is great for transporting someone with a physical handicap where others have to help them get in and out of a vehicle ... but it is hard!
ali