I looked for the same thing few years back.
The good DC SSR's and mechanical relays with snubbers were more expensive than I wanted to pay.
You could use a cheap square wave inverter and turn that off and on with an SSR.
Check this tread out:
Water Heating Controlled by a Classic 150
Maybe you could rig something up with a snap switch attached to the tank and use a SSR to turn the element off and on?