hmm, not so sure you can buy just the brackets. From that exploded diagram it appears that they may just bolt in and are removable. so maybe ya can get lucky and find a metric ton in a jy. I've never looked at mine close enough though. I'm sure somebody will chime in.
While we're on the topic, I will have a pair of rust free doors with those factory mirror mounts that I will be getting rid of in the near future when I swap my doors out for full power. If anybody is interested and really wants that style mirrors, let me know.