Office chair?

Swiss ball?

An inexpensive possibility. I know this 20 yr old standard rolling desk chair isn't doing me any good.

Yes, Swis Ball is a great idea for an offeic chair, its inexpensive also, but if you cant afford it & you want to stick with the chair, then you can buy the chair which has a good support for your back, so that your ears, shoulders & hips remain in straight line while sitting, you can prefer that chair, but still if u can get swiss ball, it will be great for your core muscles.

I ditched the chair completely, I raised my desk and stand all day instead.  Swiss Ball would be the next best thing.