Where can I get a replacement heating element?

Th easy fix would be installing a new heating element that has burned out. You don't say what size this tankless heater but if it stopped working entirely you might have just one heating element inside. You can test it by (1) turning off the circuit serving the heater, disconnecting one of the wires to the heating element, then taking an ohmmeter to see what the resistance of the element is. If it is "infinity" then it is open and burned out. Replace element. If it is around 6 ohms (for a 10kW heater running at 240V) then the heating element is OK. Unfortunately those heating elements don't last forever. Check the repair manual on how to replace the heating element.

Won't matter much... except long elements generally burn out less often than short ones since heat is spread across larger surface area .. assuming both elements are same wattage.
Lower wattage element will heat water less quickly by a few gallons per hour.
http://waterheatertimer.org/How-to-replace-water-heater-element.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

follow this test and use the multimeter and fix it. God bless you
If there is no hot water, first push the reset button on the high-temperature cutoff; test the thermostat and replace if faulty (see below). Test the heating element(s) and replace if faulty
.Test and replace a heating element:

1. Turn off power at the electric panel and post a sign warning others not to turn it on.
   
2. Remove the access panel on the electric water heater.
   
3. Disconnect one of the element wires and set a multimeter to RX1000 (resistance times 1,000). Touch one probe to an element mounting bolt and the other to each element terminal screw, in turn. If the tester displays anything but infinity ( open circuit), replace the element.
   
4. If necessary, set the multimeter to RX1 and touch the probes to the terminal screws. If there is any resistance reading at all ( closed circuit), then the element is good. If not, replace it. Both upper and lower elements are tested in the same manner.
   
5. To remove the element, first drain the heater.
   
6. Disconnect the remaining element wire. Remove the mounting bolts holding the element in place. Remove the element.
   
7. Replace the heating element with one of the same model, shape, and rating. Make sure you also replace any installation gaskets.
   

follow this steps and fix it. God bless you
Test and replace a heating element:

1. Turn off power at the electrical service panel and post a sign warning others not to turn it on.
   
2. Remove the access panel on the electric water heater.
   
3. Disconnect one of the element wires and set a multimeter to RX1000 (resistance times 1,000). Touch one probe to an element mounting bolt and the other to each element terminal screw, in turn. If the tester displays anything but infinity ( open circuit), replace the element.
   
4. If necessary, set the multimeter to RX1 and touch the probes to the terminal screws. If there is any resistance reading at all ( closed circuit), then the element is good. If not, replace it. Both upper and lower elements are tested in the same manner.
   
5. To remove the element, first drain the heater.
   
6. Disconnect the remaining element wire. Remove the mounting bolts holding the element in place. Remove the element.
   
7. Replace the heating element with one of the same model, shape, and rating. Make sure you also replace any installation gaskets.
   

Replace heating elements can be found at amazon, click here and take a look: Amazon com rheem heating element . I hope this was helpful... cheers

When the top of the tank is hot the upper thermostat removes power from the upper heating element and transfers the power to the lower thermostat and heating element. If the lower thermostat is defective, then the lower portion of the tank will not be heated and the supply will be greatly reduced. You replaced both elements however check that the lower element is coming on after ther upper one cuts out and is acutally working.
Check for power at the upper thermostat terminals where the power is sent to the lower thermostat and heating element. If there is no power then the upper thermostat should be replaced. If there is power then check for power at the lower heating element. If there is no power at the lower heating element then replace the lower thermostat.
If there is power to the lower heating element then it should be getting hot. If it is not, maybe defective. Another possibility is a broken dip tube. Check for a broken dip tube and replace if necessary.

If the dip tube is broken, the incoming cold liquid can mix with the out going hot liquid and cause it to seem as though you are running out.

I am guessing this is an ele water heater and the temp setting are not the problem.
Problem: Not enough hot water or water temperature too low. When the top of the tank is hot the upper thermostat removes power from the upper heating element and transfers the power to the lower thermostat and heating element. If the lower thermostat is defective, then the lower portion of the tank will not be heated and the supply will be greatly reduced.
Check for power at the upper thermostat terminals where the power is sent to the lower thermostat and heating element. If there is no power then the upper thermostat should be replaced. If there is power then check for power at the lower heating element. If there is no power at the lower heating element then replace the lower thermostat.
If there is power to the lower heating element then it should be getting hot. If it is not, replace it. Another possibility is a broken dip tube. Check for a broken dip tube and replace if necessary.

The dip tub is located under the fitting where the hot water comes out.

you may have a bad grand or the element is not heating

I would say then the heating element has a resistive short to ground. The 1.98m Ohm reading is high, but not high enough -- should be infinite. I would say replace heating element... And, when you install new one, make sure there is no water leaking thru heating element cavity and ensure area around cavity is clean & "in order".

How old is water heater?

Charlie

Well by replace the top element is proable not the problem.You see the top element heats the water at the top first and then the bottom take over.So, my guess is the bottom element is bad.Replace it and I think your troubles will over.