I recently broke my Hermin Miller Aeron chair armrest bolt and I replaced it for less than 2 dollars. I was about to start watching some TheVimeagen hack on some HTMX and my left armrest broke and fell to the ground. Upon examining it, the bolt broke.
WTF. It's likely a 15-20 year chair, but still.
I was upset as I was feeling in the mood to start coding up something (something that I haven't gotten a chance to do lately) and this seemed interesting to watch, follow, and try myself. I was upset and went straight to Amazon to find a replacement. To my surprise, the price was about $14.99. DiWHY to the rescue. I started looking through Youtube and found a couple different videos.
Through the videos and the comments I found that you can use a couple different bolts. I'll save you the time, you'll need to find a 1/4-28 2.5" button head bolt. I went to Ace Hardware (US store, in Ohio, maybe in other regions as well?) in the morning but couldn't find a button head bolt that matched the length I needed. I went with a hex bolt instead and it worked perfect.
EDIT: here is the hex bolt that I used.
Hex bolt: 1/4-28 2.5" hex bolt