I'm rewriting this post--I didn't like the wording of my last one...
Advertise in the "wanteds" sections of your local Craigslist.org and Freecycle.org (if you have one.) When people die at home, many times 02 concentrators are left behind, and the heirs have no idea what to do with them, so they stuff them away in storage where they languish, unused and unappreciated.
I've found two people who were more than happy to give me their concentrators at no charge at all, and they were really happy to have found someone who could them. (Unfortunately, all of mine now need repair--that's why I'm not torching at all these days--and why I'm advertising for another one or two machines...)
Good luck! Margi