If you are looking for authentic looking WWII decals, rank and insignia for helmets: American, French, Italian, Dutch, British, Russian, misc European and German check out
I use them and they're awesome- check them out. John is a great guy and would appreciate being your source for your helmet decals. He has some extremely difficult to find WWII decals. It's cool just to hunt through them. Anyone who is a WWII buff would love this site.
He also carries some other militaria- hats/helmets & liners.