As Creative Commons licensing is more widely used, locating materials has become easier. Google, for instance, allows users to filter by license as part of its advanced image search.  Look under the Tools Setting for Usage Rights filter.

Similarly, you can filter a YouTube video search by Creative Commons. Look for it under the Filters at the top of the search page. 

The photo sharing site Flickr has also built in a licensing filter. It is available from the main search page. 

Perhaps one of the best places to start to look for media resources is Wikipedia's media repository called the Wikimedia Commons. All the materials are copyright friendly.