It is always best to use work that you yourself created. The next best method is searching copyright free (Creative Commons) material!
If you don't want to create your own digital materials then use:
Creative commons - a great collection of images, music, etc.!
Go to http://creativecommons.org/ and find the "Explore" box
Enter your search query & select options that you can modify, adapt, or build on
Make sure to give credit for your source.