I went to a town auction a couple weeks ago, and bought four compressed air tanks for $25. Now sure how many cubic feet, but they were a standard size, about five feet tall. One said "customer owned", one said "welding supply company" or similar, and two were blank. Today I took them in to the welding shop I usually go to, which happened to be the same one whose label was on the tanks. I wanted to trade them all for one C02/Ar tank for my mig.
The shop told me that three of the tanks were theirs, and they were keeping them (in a nice way). For the fourth tank they traded me straight up for another tank, although I had to spend another $30 for the Stargon in it.
This shop has treated me fairly in the past. When I brought in my broken autodarkening lens (which I had bought elsewhere) they waranteed it for free. So for $55 I got a much bigger than my (now) puny 60 cubic foot. This pretty much guarantees that I won't run out of gas on a weekend ever again.