If you did not include a model # XXXXX your question please do on you second reply if possible,this helps us serve you faster.
is this a counter top or over the stove microwave? the model # XXXXX't comming up,
CPSC Document #5061
they show the magnetron still available but say the transformer has been discontinued, it may still be available with some research, I recommend having a tech work on this however, or replace the microwave, the parts are very expensive. thanks Bryan