Yes, sir!
The easiest ones to find are on .gov websites. That is especially handy if looking for military people, guns, and other equipment. There are also a tonne of other non-military .gov websites that Uncle Sam is paying for... and by Uncle Sam, that means you! Almost anything visual that you see on those Social Security, Homelamd Security, California Forest Defense, whatever sites are up for grabs.
They are cleared for PD as well... though they usually ask for listing in the credits.
Oh, the .mil ones are even better. Many of them are the same as .gov ones, but they tend to have higher resolution images to use by clicking the link. Those are the ones that are best to use for video, etc.
Archive.org has an incredible amount of material on it. You need to check more carefully for useage notes, but a great percent is available for use. Prellinger Archives (within Archive.org) is almost all PD with very few exceptions. Pics, movies, audio... so much to go through.