I found it on Amazon:
http://www.amazon.com/The-Gypsy-Quil...dp/B002NHFHQA/
Then I found that there are quite a few different discs for balancing on, for chairs, for isometric exercises, etc.
I'd be very interested to find out what works well for a chair. I spend so much time in one, seems to me it would be good to work my core a little with one of these. (I don't think I can use a balance ball as a chair because of my knee problems.)