I've answered a question on the Quora platform, where people ask the general public about anything and everything. This is the question I responded to:
What Type of Office Chair is Good for Posture?
No chair is specifically good for posture, it's not the chair that does the posture, but you. The question isn't about the chair, but about you, your behavior.
Despite being a billion dollar industry you might be surprised to learn that there's no scientific basis for ergonomics, and the reason for this is that using yourself with poise has little to do with inanimate objects.
That said, some chairs are distinctly unhelpful in their design, so you'll obviously want to avoid those. In my opinion, that would include any chair with a mesh seat that doesn't provide a solid base for the sit-bones on the underside of your pelvis. A piano stool is as functional as a seat needs to be to allow you to have the same poise/posture in sitting as you do in standing (assuming you're capable of such in standing). For the spine, sitting and standing are essentially the same thing, you don't "sit down", but "stand up" on your bottom, using your sit-bones as the new "heals" to support you.
Like standing, sitting is an activity for your spine/torso, an act of not falling over, which is a movement. Posture isn't a position, it's a movement.
My guess is that you're not actually asking about sitting anyway, but reclining against a backrest. I very much distinguish between the two, sitting is an activity and reclining is being at rest. A legitimate question might be is it even possible to have a posture when at rest?! If you're well supported in being at rest does it matter what you're posture/shape is?
The main thing to ensure when reclining is that you hinge the whole spine back from the hip joints whilst maintaining contact with the sit-bones, and not rolling back onto your coccyx.
I have an office chair which I bought from Amazon for about £80, and it's perfectly functional. Don't needlessly waste money on expensive furniture when a little self awareness and common sense is the answer.