Again, Lake City and Winchester are the primary suppliers of M855. Winchester does not sell M855 commercially, though they do make it available to police departments.
IMI M855 is available from several sources and other foreign surplus ammo is available from time to time. Recent brands have been Santa Barbara from Spain, Sellier & Bellot from the Czech Republic and Radway Green SS-109 from England. It should be noted, though that the Radway Green ammo is underloaded specifically to function in their horrible L-85A (SA80) bullpup rifle and may not cycle in other rifles.