What type of chair are you using now? If price is not a problem, why don’t you just get a new chair?
A few months back my computer chair broke, and being short on cash, I got a cheap one to replace it. It has a straight back and seat, and not very much padding. My back’s been killing me since I got it. But I got a little more money recently and decided to spring for a “premium managers chair” at Wallmart. It was a little more than I wanted to pay ($99), but as soon as I sat in it I knew I’d made the right decision. The padding on it is very thick and comfortable, and the lower back padding juts out a little bit, so it supports the small of your back. I can sit in this new one for hours without any problems.
Sorry I can’t offer any advice on cushions, other than if your chair is making you ache, you’re probably best getting another chair. If you can afford to spend a bit more (over $100), you can get something really nice. If you spend that many hours a day in it, the investment will be well worth it.