Public Domain Music?

[Rebecca Danger]

Does anyone know of a source of public domain music that is free for commercial use?  More specifically, if there is any that is swing or big band?  I think pretty much anything recorded before 1922 is public domain, but I imagine I'd be finding more ragtime than swing if that's all that's available. 

I guess my main question is how can I find a swing or big band song to play in the background of a video (that people will be paying for), that doesn't cost me any money, and so that I don't get sued? lol  Any ideas?