Winchester only sells new-production M855 ammo to LEOs or to the military. You aren't likely to find any in bulk. At best, you might find a couple of boxes at a gunshow or something.www.ammoman.com
often has surplussed Win M855, but you should consider it blasting ammo, as you never know why it was surplussed. Often, surplussed ammo was flawed in some way from the factory. Otherwise, ammo is surplussed when it is obsolete due to a caliber change (which won't be 5.56) or because the ammo was deployed and was exposed to harsh field conditions (i.e., high heat).