Hi Amy,
There are some free services out there, but the one I find the most helpful is Audio Blocks--it might be called Story Blocks now, and if you have a Techsmith subscription for Camtasia, Audio Blocks is included. I think it's $99 a year--something like that, and it has all kinds of music and sound effects. I like to use audio a lot just for fun, so it's a good investment for me.
Someone recently recommended FreeSound.org, and I used that to find some flamenco castanet sounds. 🙂
Good luck with your search!
Jodi Sansone
(203) 451-5421
jodi@jodisansone.com