its probably WAY to much hassle but
www.back-shop.com makes custom made chairs. i have one and it kicks ass. no more back pain. perfect posture.
only thing is you would need to take a trip to Dublin to get fitted for one, they cost about 1k and shiping would probably be another couple hundred. i'm not sure if you're that desperate though or if your back is that bad, but if it is, go there, they will sort you out.