Where can I order a replacement heating element for a Primo 900135 Model YL1335S?

SOURCE: no hot water

If the water does not heat, then you have to troubleshoot to find the problem. With a voltmeter determine if 240VAC is actually getting to the water heater. Take care here - a defective circuit breaker can give a false 240 volt reading - check for the 240 volts across the upper element (i.e. when the 240 volts has a live load on it, not just an open circuit test).
Also, remember, the upper element has priority over the lower element, and if the water is cold, the upper element will try to turn on and this locks out the lower element (only one element is allowed to heat at any given time). The lower element comes on ONLY after the upper thermostat is satisfied. Therefore if the upper heating element is burned out you will never get any hot water. If you suspect this, TURN OFF THE POWER TO THE HEATER and take a resistance check of the upper element.

SOURCE: My Rheem hot water heater stopped putting out hot

If you are not familiar with the element, I would get someone that is familiar with one and have them do it. It's not something I would recommend you do based on your question. There's electrical, pressurized water tank, and a whole lot of water that may cause you to have a worse bad day.

SOURCE: no hot water when heating tempreture reached

I have a triton opal shower.

I turn shower on low pressure and cold water. then no low pressure and piping hot water for a very short time then low pressure and cold water.

I've changed the solenoid and the little black plastic ball.

Help !

SOURCE: I replaced the heating elements(2) and

Residential water heaters USUALLY follow same diagram unless water heater is wired for other purpose. Multiple diagrams on instructions show available options. Take photo of wiring before removing wires on any electrical device.
Upper element.
Black wire from top of heater connects to #1 top left screw.
Red wire from top connects to #3 top right screw.
Blue wire from upper element connects to #4 middle screw on right side
Red wire from lower element connects to #4 middle screw on right side
Yellow wire from upper element connects to #2 lower left screw
Black wire from lower thermostat connects to #4 lower right screw
Upper element: Yellow and Blue wires connect to either screw
Lower element: Red wire from top connects to either screw
Lower thermostat: Black wire from top connects to top screw
Short wire between lower thermostat and lower element

http://waterheatertimer.org/images/Water-heater-wiring-w-numbe.jpg
http://waterheatertimer.org/images/Thermostats-numbered-divided-500.jpg
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html

SOURCE: 1992 Reliance electric water heater model 501 has

Check the breaker and top thermostat and make sure they are not tripped. Move onto replacing elements if above is not the problem.
Turn water and power off to heater and attach a water hose to bottom of heater to drain down tank. May have to open hot side on a facuet to get it to drain.
Once drained remove elements with 1 1/2" found at local hardware stores. once elements removed and new ones in place. Turn water on and get the air out of the system. Once water is flowing from the hot side from all of facuet's in house.
Once all air is out now yaou are resady to turn power onto heater.