With Monkey Mirrors there is an ingeniuos helmet mirror using a mounting made out of stainless steel available in Germany.
This mirror will not be fixed using an adhesive pad or similar as usual but will be kept in place by the friktion between the plastic coated stainless steel wire and your helmet. Mounting is done within some minutes and due to the rigid construction it will not move even on cobble stones.
In addition Monkey Mirrors uses a quite big and optical high quality mirror. This enables the user to keep track about the traffic behind in all circumstances.
You will find a detailed review on bentrideronline