For the machine I use most I have an old office armless chair that rolls on 5 casters. The back and height are adjustable.
It take some testing to find one that's just right. Look for lumbar support, especially if that's where your pain is. There are lots of choices out there. Maybe some stores will have an exchange policy so that if your new chair doesn't work out for you, you can trade it back for a different one, but there might be a restocking fee.
If you have carpet, get a protective mat to roll or slide on. Being able to roll with ease might make a difference to your back. Good luck!