You can search for material under the Creative commons search engine. You can add the CC Search Browser Extension, to your Firefox, Chrome or Opera browser, which allows you to search for CC-licensed images, download them, and attribute the owner/creator with fewer clicks. See Introducing the CC Search browser.
You can modify the Google and other search engines to look for creative commons material only. In advanced search you can set a filter for reuse. Go to Google Advanced Search. Then at the bottom of the page to 'usage rights' and change it to the type of use you want. For example ''Free to use, share or modify" would be appropriate if you are changing the work, 'free to use, even commercially' would be appropriate if you want to use the material for promotion or short courses
See also where to find CC licensed material.