[estevesm]

Here's the conclusion to the story:
I called GE to see if they could send me the mag tube. They said they can't send it to ordinary people like me, only a tech can replace it. Then, they charge $80 for the visit.
They declined to let me know what the repair would cost (labor only), so I had to schedule an appointment. Good thing is, the service guy called me before coming and told me it would cost me $180 total to replace the mag tube.
Then I said "thanks, but no thanks" and bought a samsung microwave for 230 including shipping and tax. (same cu ft, same wattage, similar features).
I'll have to do the install myself, but 180 to replace a part under warranty sounded a little bit too much.
Thanks all for the replies!