In addition to your interviews and B-Roll, you will need to find footage that is historically relevant to your demographic and story. Below are some resources you will most definitely want to use to legally acquire footage for your docs.
First, watch this animation on Copyright Law and Fair Use to protect yourself, me, the school, and the school district in your choices of footage. This is not optional.
For copyrighted photos and video you use under Fair Use argument, here is a best practices guide to attribution. Also not optional. You MUST attribute the author and source of all copyrighted material used in your documentary.