Where can I get the door mirror mount thing

[Field and Son]

I went to mount a pair of spor mirrors on my truck today and realized I am missing the part inside the door with door that takes the three bolts from the mirror in a trinagular bolt pattern. Not sure how you would get one back in and hold in place to mount the mirrors, but I am sure it must be able to get done. I only need one of these and the three bolts. Any ideas??