Hey guys,
here is a link I found which - as far as I can see - has most flying helmet options available for use - both civil and military.
It has info on Each type and links to Manufacturers and Shops, mostly European from what I can see.
Hope it helps.
Flightgear On-Line, the website for the collector of military flightgear
p.s some of the links are broken but it'll give a reasonable overview, Alpha helmets are listed under "RAF Helmets".