Kim,
I have a steel tank too. You want to make sure that a fitting or broken water line isn't your problem before searching for a new tank. It is more likely that a copper pipe has broken/split than the tank breaking. Another source of leaking is the pump. If the diaphragm tears the pump will leak. I would remove all of the sogged insulation and inspect everything diligently.
Repairs: Steel water expansion and storage tanks can be purchased at plumbing supply distributors or, depending on size and configuration, at the local big-box or farm stores.
Another way to work this problem is to order a new plastic water tank and a supply pump. You will have some plumbing to accomplish if doing it like this since drains and fill fittings may be in different locations.
Picture posting information is in a sticky thread in the DIY section of this Forum.
Post back with what you find and the kind of skills you have so that reasonable advice can be given. Best of luck with your repair.