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Go on Amazon and search "pouf ottoman" - they have tons. You could also do a regular ottoman that's sturdy enough to sit on, or a floor cushion or something.
You could also look into gaming chairs (the floor kind like this) if you really want to get into gaming, and that looks like it would be a decent height for playing board games on a coffee table as well.
This is the chair I have for gaming.
I know it is different from an office style chair, but I love this fucking thing. I dont even use the built in speakers for it. I do wish it had legs, but even without them I find it to be comfortable. I will warn you that leaning too far back will cause you to tip, but it is more funny that harmful.