'68 Coronet R/T
Oldtimer
I would say used will be your only option, I've never seen one of those plates for sale. The one like Sprint posted is the only one I can remember seeing in a parts book. I checked my parts trucks, and none of them have that reinforcement plate.
(As a side note, I don't think that is the original mirror, it was most likely like the one in the diagram I posted.)
I agree with you on the mirror being wrong.
The original door on the passenger side had this:
The owner wants to go with the newer style mirrors and the driver's door mirror looked to him like the better option because of the 2 screw mounting.