Jeff
There are a lot of recycle places around these days. They come up with all kinds of cool stuff including air tanks. Talk to them and let them know what you're looking for. Maybe they will call you if one comes in.
You can buy old Propane tanks at the gas company for scrap price most of the time.
Do your homework! before you start remove any pressure first thing. Remove the valves and clean it out good
before you do anything else.
I have had "The Tank" now for 18-20 years. The bottom of the tank is getting thinner now and I noticed a pinhole leak between the tank and the fire box which will be fixed with a new bead around it. I soon (next couple years) will have to weld a new bottom in it. It has turned out many many fun meals.
I think you did pretty damn good on both counts. Buying the pit, and the PUI(Posting Under the Influence)! 
I want a stick burner something awful, but getting a good USA made one shipped to Alaska is crazy expensive. Just been watching craigslist until I find a cheap air compressor or propane tank of the proper size, and roll my own.
Good luck with the offset, and look forward to your cook reports!!
Jeff
Hey Jeff, stay diligent and watch CL constantly......something will pop up. Just insist on 1/4 inch. I know you can get by with 3/16 but 1/4 makes all the difference.
Once my pit got to 250 and I needed to add wood, I had to add smaller splits because that thick metal was holding the heat in and I did not need a large fire.
You will actually use more wood with thinner metal pits.
Appreciate the advice, and I will heed it! I think a 60-80 gallon air compressor should be close to 1/4" thick. I intend to do a reverse offset design, so the extra plate inside should add additional thermal mass to it as well.
Jeff
