If you can't find a tank just get your local steel shop to roll you a tube in 6mm plate. Probably cost you less as well. And if you keep on the right side of them when you cut out your doors you can get them to run the doors through the roller to match them to your tube.
Post pics of your build.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford