To create a B-roll gallery you and your students should be taking pictures, recording videos, and capturing audio throughout the school year. Do this for a couple of school years and you will build a useful gallery of B-roll media.
Tools for creating a B-roll gallery:
This list can get you started, but it's just the tip of iceberg when you consider how many web and mobile apps are available to you and your students.
Aviary's Mobile image editor is a free app for iOS and Android devices. The Aviary mobile app allows you to quickly crop images and add effects to your images. What I like about Aviary's mobile app is that unlike Instagram you don't have to register or join any social networks.
To capture quick audio recordings try AudioBoo for Android and iOS devices.
To organize your gallery of B-roll media try using DropItToMe to have students contribute to a DropBox folder. If you're working in a Google Apps for Education environment use shared folders in Google Drive.