3rd Eye make a couple of clever head-mounting mirrors – one with a sticky pad to attach to your helmet, and one which clips onto any pair of glasses. Both have a small mirror mounted on a ball-and-socket joint.
Busch & Muller Cyclestar mirrors are very solidly-made and very adaptable.
They all have the same round convex mirror, which is practically unbreakable and attached with a ball-and-socket joint to a choice of different stalks.
The stalks are all made out of steel for strength, and have another ball-and-socket joint where they attach to the mounting bracket. Two mounting brackets are included, for attaching around the handlebar or inside the end.
Also available is a bar-end-only mirror, which is very compact…