I have used bike and helmet mirrors. I like them both but since I ride more then one bike, the helmet mirror ended up being easier.

The one I have been using for a long time is https://hubbubonline.com A small supplier that makes a durable product.

BTW, I started with a mirror that was great on my bar end shift bike. After long use, I needed to replace but it was no longer available given the change in shift design. But given what I said above, helmet mount is my preference