Repairing side mirror damage

Hey guys, a couple of days ago I was minding my own business driving down the interstate, listening to some kicking 80's music when a car 15 feet in front of me in the fast lane hit a piece of a blown semi tire. The projectile hit my front quarter panel (creating a divot straight through the clear coat) then bounced into my driver side mirror. I didn't realize any damage at first, but when I got out I noticed it had knocked a piece of the bottom of the mirror. I took it to the dealership to see if they had a spare or could sell me that one piece and of course they wanted $350 or so for a whole new mirror. Everything with the mirror works fine. I just need the bottom plate. Any ideas?