I have started building cold smokers as well. here is the first model nearly ready to go. I also got onto a supply of over 1000 propane tanks for cheap. Looks like I am going to be busy for a while.
This is the place that re certify's them. These are the ones that failed or they started to sandblast them and found pits in them.
I can get 250 Gallon ones for $100 There's about 4 acres of these tanks laying around.
My next plan is to make some trailer smokers. I just need to figure out how I can use these tanks to make the firebox as steel plate is expensive here.