I've seen helmet mirrors in
Evans. On a whim I picked up a handlebar mirror there a couple of days ago, but I kind of wish I had got the helmet one instead. Although the bar mirror isn't too bad, I didn't find its field of view very good, and it's sufficiently far out of my field of view that when I'm looking forward I'm unlikely to spot something that appears suddenly inside it. Although it is quicker than turning my whole head around to look back, it's not that much quicker.
Also, as it fits on to the end of the handlebar, it does make the bike wider.
It's this one, by the way, by Blackburn:
Although I don't have it sticking straight out like that.