I work for an ergonomic furniture company that sells office seating and i use their Saddle seat with a tall cylinder and foot ring. Don't be fooled by the lack of back on this think. I can sit in that seat for 8 hours and have no issues with my back.
www.humanscale.com go to products and services and then to seating and then look for the saddle and the pony. I use the saddle well ..because i have a fat A** lol
they are a bit expensive from buying on the website, but look at the top for the where to buy and use your zip code to see if there is a retailer near you.
Becky