I bought my rear view mirror from my auto supply store. O'Rilleys.
It has just the right amount of curvature to the glass to give a wide rear scan.
It is appx 10" long and 2.5" tall. On my Avid MK4 I have it positioned where the top glass and the windshield meet. It is a tail dragger and I can see aircraft taxing behind me and I can see if my smoke generator is working.
40% of mid airs happen with aircraft coming from high and behind.
I am also using Stratux with IFLY GPS on my Android tablet and have already avoided some VERY close calls. When I see a target overtaking me from behind on my adsb I check the mirror and hit the smoke button.
The mirror is designed to clip on to your rear view mirror in your car, but with some minor modifications you can attach it to your aircraft.
Many of the world war 2 fighter aircraft had a rear view mirror and even today modern jet fighters have them.
Fighter pilots have a saying, "It's the one you don't see that will get you every time."