You don't need to buy new - check your cupboards for vintage quilts, plaid blankets, or even scarves to drape over chairs. If you're handy with a sewing machine, you could even make your own cushion ...
I've sat on chairs for longer than I can remember, and have been doing so in a professional capacity for many years, too. In fact I've probably sat in more gaming chairs than any person on PC Gamer.