It's not a 100% guaranteed source for video, but on YouTube you can do a search for whatever video you need and then refine your search filters for 'Creative Commons.' These are videos that the creator has designated as free to use for anyone. They aren't always the best videos, and you need to be careful because if that creator used footage that they were not allowed to it can slip through the cracks and you can still be liable for using it. But this is where I typically go when I need additional b-roll.