Do you work in a job where they force you to stand when sitting would make no difference, despite it being bad for you? This is how I got to sit when nobody else did.
I walked into a local clinic and told them my foot hurt, and that I was standing for 50hours a week. I told them I had the exact symptoms of plantar fasciitis, shooting pain in the heal when ever I walked. I said it was sensitive when the doc touched it, an xray reviled some inflammation. The doc wrote a quick note and then for a the next year I had a chair when everyone else had to stand. Please not that nothing i did was a technically even a lie. The entire thing after insurance(its a USA thing) cost me like 50$. the best thing about it is the company i work for got me a chair I got to keep worth 2x that amount.
It puts them in a tricky spot, if they deny you a chair then that's not doing a reasonable accommodation, and a lawsuit waiting to happen.