This subject has been beaten to death (no pun) in successive threads on this forum.
For a start, look at the last few pages of this thread:
http://www.pprune.org/forums/showthr...flight+helmets
If that's no good, familiarize yourself with the search tool and you will find two more threads (at least) that deal with flying helmets (which and how much, etc.).
http://www.pprune.org/forums/search....archid=1863520