If there is no power at all then check also the door switch and thermal cutoff.
If it is working but not heating, then test the high voltage
capacitor,the rectifier diode.or the magnetron. If all this are fine,
test the control board.
capacitors can be tested. The capacitor must be allowed discharge before
testing, or you risk to get electrocuted, evenif power is off.Capacitors
can get discharged by shorting their two contacts.
Before replacing the magnetron test also the control board.
Replace the magnetron if the checks and all of the high voltage component tests are good, but the unit still does not heat.
All locations, and description of this parts can be located using the diagram here: Counter top
Parts must be tested to find out what is wrong.
Remember that microwaves are potentially dangerous devices if not repaired properly, the job must be done by a competent person.
A microwave energy leakage test must always be performed when the oven is serviced for any reason.
Regarding parts, contact the seller here to purchase parts.
You can also order from MicrowaveSvc , they also provide parts.