Everything listed on this page:
556x45 report.
It's updated hourly. Just click the "Vendor" link to go directly to the vendor's web page. I've been watching 556RECON for about a week, and vendors with stock come and go hour-to-hour. Vendors are also listed in the right hand column.
Right now I'm seeing Ables Ammo, Ammo.net, BulkAmmo, Cheaper Than Dirt, iAmmo, LAX, Palmetto and SurplusAmmo.