You can go about this in several ways, such as through an advanced search on Google or Yahoo! where you can specify by permissions for reuse, or search directly at music platforms and libraries that host CC-licensed music and music that is in the public domain. 

Here is CC's search tool, which is not a search engine, but a portal to other search services and platforms that have enabled searching by CC licenses: http://search.creativecommons.org.

CC also features pages on the various music communities and record labels that employ CC licenses:

• Search music communities for CC-licensed music: http://creativecommons.org/music-communities.
• Check out record labels under CC licenses: http://creativecommons.org/record-labels
• Vimeo, the high quality video sharing platform, also has a Vimeo music store for those seeking music for their videos: http://vimeo.com/musicstore. The store has free CC-licensed tracks in addition to tracks you can pay for.

These are just a few examples of places you can look for openly licensed music. Be aware that there are six different CC licenses, and other open content licenses to boot, and that they are not all the same in their conditions for reuse. Also, "Under CC licenses, syncing music in timed-relation with a moving image is always considered an adaptation," so you would have to find music that is under a CC license that allows derivatives.

For more information on the CC licenses, see http://creativecommons.org/licenses.