Hello: Have a look here: http://www.htproducts.com/ They have both gas and electric heaters. Another and totally different concept is the Howard Harris heater, http://www.macraesbluebook.com/search/company.cfm?company=833000 . I’m not certain their status as Mr. Harris recently passed away, but for an electric heater, his design has merit. It’s unpressurized plastic and doesn’t concern itself with water quality.
Stainless heaters need to be done well and of the right grade of stainless or they can fail even faster than regular glass lined steel.