I have three different handlebar end mirrors currently in use:

Sprintech: https://www.biketiresdirect.com/prod...dropbar-mirror
Italian Road Bike Mirror: https://www.ebay.com/itm/392744672273
Spurcycle R50: https://www.spurcycle.com/products/r50-mirror

All are about the same price (though Spurcyle was offering theirs for 50% off a few weeks back; I bought two). I have the Sprintech on several bikes and like it a lot. The Italian Road Mirror is a bit finicky to set up but less obtrusive looking. I haven't actually installed the Spurcycle one yet but am planning to do so soon. I've also had the Berthoud before it got damaged by Amtrak baggage handlers; it's a tiny thing, not particularly great for seeing what's behind you. I also typically ride with a Garmin Raid and really like the combo of the mirror and the alert from the Raid to be fully aware of vehicles overtaking me. Like others in this thread, at age 64, I have pretty limited flexibility to look over my left shoulder, so the mirror is essential.