Where can I find free, non-copy written music?

I have a cute, short video of one of my grand daughters, who is planning to follow in Evel Knievel’s steps. She scares the shit out of me.
Anyway, I added circus music to the video, and it was perfect. But, when I tried to post it on FB, FB yelled “NO!!! PARTS OF THIS VIDEO ARE COPY WRITTEN!!!”
So I stripped the music off and reposted.

So…where can I find music I can use legally?