Banker's Chair hardware?

I've wanted a nice bankers chair since I was young. With access to amazing cnc equipment and 3d resources at my disposal at school. I've decided it would be pretty sweet to make my own. However, I am having the hardest time coming across the hardware. Pottery Barn sells a newer model of these chairs in the $400 range but they all have a pneumatic cylinder instead of the classic spring and knob. I've watched craigslist but these just don't seem to pop up that often. Any input would be helpful, thanks

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