To echo the two posts that make the most economical sense--Find the used office furniture market in your area, and go from there. I picked up a mint-condition Herman Miller Aeron for a hundred bucks from a dealer who got a contract to refurbish an office. Said office was run by idiots who had no idea that the old boss had purchased an $800.00+ chair for himself, and let it go with the rest of the furniture that was headed to the dump. The dealer caught that, saved the chair, and then realized he had no storage room for it, so put it up on Craigslist.
You'd be surprised what you can find through that same sort of source. Give the office furniture dealers a call in your area, and not just the guys who specialize in used stuff.
Also, get a timer of some sort so you're reminded to get up at some interval and do something physical like push-ups or sit-ups. I put up a chin-up bar and TRX trainer anchor in my new office, and it makes a hell of a difference when you start doing PT in between your intervals on the computer.