Plenty abound in FO3 but FNV tough as nails...never they new existed. Wiki said the Great Khan Armory and Quartermaster Bardon would occasionally have in inventory. Sure after 12+ hours of real time gameplay QM Bardon (and only NPC for me anyway) finally coughed them up. I did this by relieving Bardon of all apparel each visit using the weapons exploit so it did not cost anything (XBOX). Waited 3 game days and confronted Bardon again to see the newly respawned inventory. I noticed saving before entry in to the underground part of Hoover Dam did not change the content of the inventory. Perhaps going at differrent times of the day would generate the probability of all raider helmets more often. This is unconfirmed as I did this towards the end to obtain the Blastermaster (Guns +5, Expolsive +5) helmet as I was getting bored to the point of defeat.

Cook-Cook has the final Raider Arclight Helmet for those Hoarders out there, like myself.