Any office chair can be converted into a reasonable chair with a McKenzie D roll (we sell them, $44).

But for those with more significant back problems who wish to get a high quality chair I recently came across this website. It has adjustable segments at the back of the chair to increase/decrease the curvature to fit your spine.

http://www.bluehen.com/allseating-cpod-chair-p-727.html

 

I recommend the Cpod (and am ordering one for myself).

Costs about $620 on special now. They ship to Australia for about $130.