It's called "first-surface reflective glass" -- if you try, you can find it. This place will cut it for you, there's also engineering and optical places on the net that say they'll do it. Now, I do know there are different grades of mirror glass. Just like diamonds the better stuff will cost you more. That's about as far as my technical knowledge goes.
https://robinsonglass.com/auto-glass/sideview-mirrors/