On my biking glasses I have a similar mirror, being essential when riding. And as I've got used to having it there, I put one on the inside of my sunglasses.
On a pivot, they are stuck with double-sided tape onto the glass (plastic / whatever), and set-up. I super-glued the pivot so that it wouldn't change position when cleaning the glasses.
They can be purchased quite cheaply from Cambriabike.com
