I wish the bungee chair people made mattresses.
You don't specify exactly how you're uncomfortable. If you can't find an office-type chair that works for you, maybe try alternatives.
The ball chair someone mentioned earlier is affordable (I found them listed online in the $65 price range).
Another one that I haven't seen mentioned yet is the saddle chair.
https://en.wikipedia.org/wiki/Saddle_chair
I have three quilting friends who each own a saddle chair, and they all love it. It's a bit more expensive, about the same price as a good office chair, but they all say it's well worth the price.
Good luck in your search.