You can find a large number of sound effects and music beds in iMovie. These are all available to you and are royalty free.
However, if you need additional music and sound effects, there are several sources. This is not an exhaustive list, so if you know other good sources, please add them!
For sound effects, try http://www.freesound.org/
For music, try
For Video Clips, try
http://archive.org/details/prelinger The Prelinger Archives
If you use Creative Commons material, it is important to understand the licensing. See this article.
Note that royalty free does not necessarily mean free. It just means that you might have to pay one time, rather than pay by volume every time your video is viewed. Many of the links above are free, but there may be some charge for some.
It is also possible to create your own music tracks in GarageBand or Logic Pro. This is free and you can make exactly what you want.