One of the leading organizations supporting copyright friendly material is Creative Commons (see About Creative Commons Licenses and the video below). A good way to find these sources is by using an internet search engine like Google. When you search for what you are looking for, also include the words "creative commons" to your Google search.
For Example - Let's say you are looking for an image of a microscope for cover art on your learning packet. Go to Google and search for "Microscope Creative Commons." After searching click on the word "Images" in the top left and you will be presented with many pages of images to explore. Remember to check the license restictions and to give attribution by citing your source.
Here is a list of a number or copyright friendly sources: