Hello: ISE makes tanks of stainless or bronze. They used to make copper tanks, so it likely isn’t the tank causing rust. Also, their tanks are used for an instant hot water tap.
Do you have steel plumbing or some other source of iron in the system? Is there possibly a grounding issue that is making equipment fail so fast?
Marathon makes 15 and 20 gallon plastic tanks; http://www.marathonheaters.com/cons_specs.html
This might be a good application for a small tankless electric unit.