I like this one:
https://www.amazon.com/Third-Eye-Helmet-Bicycle-Mirror/dp/B0012OKA6M/ref=sr_1_4?s=sporting-goods&ie=UTF8&qid=1466525019&sr=1-4&keywords=Third+Eye++Bicycle+Mirror
I use a bit of Goop to glue it to the helmet instead of the hook and loop stuff. Make sure you mess around with placement BEFORE permanent attachment. Getting it in the right spot is critical.