The availability varies, but I bought a boatload of .308 and .223 brass from <span style="color: #FF0000">THESE FOLKS</span> a few years ago - mostly Black Hills Match, LC, and Federal. The advantage to theirs is that it is (or at least was last time I talked to them) all shot in bolt guns at their training facility so you can avoid all the issues that come with brass shot in the over-sized chambers of machine guns.
Dealing in brass is not the main thrust of their business and you need to give them a call to check on availability, but they are great folks. I ordered several thousand pieces of each caliber and they shipped it to me as they accumulated enough to make shipping worthwhile.