Whatever you buy, make sure it's an ergonomic chair. Any true ergonomic chair should have the following characteristics:
-It should have at least three adjustment levers underneath it. One for height, angle of the seat, and angle of the backrest.
-Adjustible-height armwrests
-Adjustivble-height back
-No less than five wheeled "legs"
Most office stores have a few of them. Expect to spend $200-$300.
Cheers