The easiest place to start is the Defense Video Imagery Distribution System.https://www.dvidshub.net/ (You can eventually get here from the Defense.gov site, which is actually one of the better federal websites, but this is the most straightforward of the various subportals.)
Greg, I know you know all this, but it's always worth mentioning for the less-experienced folks who'll find this thread later, that when talking about government public domain footage: yes, if tax dollars paid to acquire the footage, it's available to you, but rights are still rights, including trademarks, copyrights, privacy, and more.
For example, military symbols, including uniforms, are trademarked, and there are very strict limitations on usage for such imagery. Commercials are out. The same holds true for individuals featured within clips. For news, documentary, etc., you're likely fine. Anything that implies that the individual speaking, or even just generally visible onscreen, in any way endorses you or your work -- not okay.
Those are a couple of broad considerations, but DVIDS has a nicely precise, easy to read discussion of this. https://www.dvidshub.net/about/copyright. They're good reminders to everyone dealing with rights issues, ie, pretty much everyone, all the time.
Lots of great stuff in these archives, though, including high resolution clips. More people should be taking advantage of these resources. After all, you're paying for them! 😎👍