There are licensed resources are readily available for professional filmmakers to pay for and use.
Note that these sources are strict with their license terms and copyright so users should carefully review the licence terms and contact them for guidance on selecting the right licence type for their needs. Some of these websites also have a "editorial use only" restriction (e.g. Shutterstock's explanation here) which prohibits commercial, advertising uses.
*The list is intended to inform students some of the resources used by documentary and film makers. It is not an endorsement of the sites. Students are advised to search for public domain and Creative Commons licensed resources instead of paying for content. Also, licensed resources especially from news agencies tend to be expensive.
Some of these companies have in-house researchers to help with locating the right material and licensing.