The only thing I could help you with is some old CO2 extinguisher tanks that would need hydroed before anyone would fill it. If I can help in that area let me know.
A welding shop that comes to my work swaps my #20 cylinder with a filled and inspected one for about $12 - $13 (creeping up due to inflation lately). So far that's the cheapest I found available around here. They sold me a brand new filled cylinder for $70 initially since I wanted my own cylinder. I wonder if I could have started the exchange program by leasing the cylinder free...