Marathon elements are specific to that water heater and can be ordered online, but generally are not at the local hardware.
Good to keep a couple spares around.
Marathon is 240Volt water heater.
Thermostats turn off one leg of the 240Volt circuit, so elements and themostats will always test for 120volts.
You removed wires off elements and tested each for continuity and tests showed elements to be good.
Test top two screws of upper thermostat and make sure you are getting 240V to the water heater.
If water heater has some hot water, then that says water heater is getting 240V, upper element is good, and remaining suspects are bad thermostats, bad lower element, or sediment build-up, or combination.
Open following link for troubleshoot run-down:http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html
If you have sediment build-up that reaches lower element, then water does not circulate around the element and water heater no longer heats very fast or accurately. If sediment is the problem, sooner or later the lower element will burn out.
Doesn't hurt to flush out the tank now and then, and vacuum out the sediment if that is problem.http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html