The Internet Archive is a great source of stock footage. We have thousands of public domain films (especially the movies in the Prelinger Archive) that can be used for any purpose, including cutting them up and just using the portions that will help your story line. You can also search the archive for video footage with other licenses that allow slightly more limited use, but may still be helpful (see this search tip FAQ about searching by license type).
We’ve noticed that several users have started uploading video that is specifically meant to be used as stock footage. These are generally short clips, often without sound, that include things like scenery, animals, time lapse videos, etc. Since stock footage is so useful to filmmakers (professionals and amateurs), we’ve made a Stock Footage Collection where these specific types of clips can be found.