FedFlix: a good resource
FedFlix is based at Archive.org, and mainly organized by National Technical Info Service. It's a huge load of Federal and State films, well-indexed and easy to search, with 'fat spots' in three distinct eras.
After 1990, it's mostly C-Span videos of congressional hearings and such. The newest I can find is only two weeks old.
Fairly sparse from 1970 to 1990.
In the '60s a big set of military recruiting and training films related to Vietnam, plus some anti-Communist school films.
Before that, it's much more random but abundant. WW2 military, plus lots of commercial and corporate propaganda that was commissioned or contributed to the Feds. Seems to start in the New Deal, not much before that.
At the moment about 7000 clips, but constantly growing at the new end from C-Span.