It sounds like you're doing an awesome job of living frugally! I hope you feel better soon.
We do have stone walls but I tend to take stone walls fairly seriously and would not cannibalize an authentic stone wall for a project. We don't have enough other loose rocks to make a properly sized fire pit.
This photo is as far as we currently have gotten, and it still needs another layer of rocks before it's high enough for me. I want to make sure no fire can get out. We live in a forest so it's important to me that it's as safe as possible.
The next step is to build a stone patio to go under it, so even occasional sprays of sparks can't set anything alight.