Do NOT cut open a tank with a torch!
I was having the same problem. I ended up going to several places that recycle old tanks and reclaim the leftover propane and after explaining my plans an assuring them I wouldn't blow myself up I got a 100 and a 200 gallon as well as a couple of forklift tanks and an invite to come back for more if I needed them. Might be worth checking places like this out.
I am amazed that these people would give you a tank after you told them what you had planned. If you had unsuccessfully cut those tanks open, your widow could have sued them.
Now what I am about to say is going to rub some people the wrong way, but so be it. Cutting open a used propane tank with a torch is the same thing as disabling a BOMB! Yes everyone has a tried and true method that worked for them or their third cousin Bubba's ex-brother in law. Why anyone would use information they found on the internet from a stranger on how to safely diffuse a bomb is beyond me.
People posting advice on how to safely do this are not doing anyone any favors. I have done a LOT of research on this subject over the years and here is what I have found. There are no written instructions on how to safely cut open a used propane tank. I have spoken to the American Welding Society and the National Propane Gas Association on this subject. Neither organization has a written standard procedure on how to safely cut open a tank with a cutting torch.
Why is this? The main reason is liability. A written manual makes the writer or the organization responsible for the information liable if someone is injured following those written instructions. In my discussion with the propane association, I was told that they do have certain recommended procedures they follow when disarming tanks and even then, tanks still blow up.
The number one cause of injury or death to welders and blacksmiths is cutting open a tank with a cutting torch. This includes new tanks as well as old tanks. If your torch is not running properly the acetylene can accumulate in the tank and ignite when more oxygen is introduced. The same thing applies when using vehicle exhaust. If the car happens to be running a little rich or has not burned all the gas before getting into the exhaust, it too can accumulate and ignite, and explode.
Filling a tank full of water provides another false sense of security. Once the initial cut is made, you will be introducing air into the tank as well as gas from the torch. Two hundred-fifty gallons of water that stinks is going to leave a mess too. Propane gas has been known to penetrate inside the top layer of the steel and explode when heated with a cutting torch.
There are just too many things that can wrong when cutting a used propane tank with a torch. Are they worth your life? The reason I am so opinionated on this subject is I lost a good friend who was a blacksmith. It wasn't a propane tank that exploded on him, just a 55 gallon drum. It was a procedure he had done a hundred times.
So my final words are these. If you find a tank, take it to a professional and have them cut it open for you. They will do a lot better job than you can do on your first try. There will be plenty of other cutting and welding to do once this is done. And don't listen to anyone's so called advice on how to safely do this at home by yourself. These people aren't doing you any favors and their "advice" may get you killed.
Beers for thought,
Juggy