Have you tried Dennis Carpenter Restoration Parts Company? If not, here is their website;
Home Page | Dennis Carpenter Ford Restoration Parts
These guys have everything you can think of and more. As an FYI, I have found parts for my 1969 conventional Kenworth W900A model, and the better side of that is (and I learned this shortly after I started using them) that Ford Motor Company, or as we all know it FOMOCO, has been engineering and producing parts for many different car and truck manufacturers like Peterbilt, Kenworth, Volvo and many others. This even includes farm tractor parts that were used by Sears and Robuck Co. since their inception along with a lot of others.
I hope this helps and you can find the mirror brackets and mirrors you want.
Best of luck!
Ps, let me know if this doesn't work, I have another outlet where you could find one but it would most like;y have to be re-chromed and glassed, that is if they would sell it to you....
I can send you a picture of the place I am talking about so you can get an idea of what they have on their acreage if you would like?