All sorts of mirrors and arms and brackets available aftermarket.
http://www.velvac.com/sites/default/files/velvac_product_catalog.pdf
Page 29 has just the adjustable long arms listed, part number 747070, and earlier in the catalog are all sorts of mirror heads.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-