Hi Michael,
YouTube has a good library of royalty free music. Personally, I subscribe to Epidemic Sound for my YouTube videos. Others like ArtList.
You can make your own music with something like GarageBand or Logic Pro (both Apple products), as well. Hopefully, some PC folks will chime in.
You can try out Adobe's available royalty free loops and remix them in Audition too.
Let me know if you need help with any of these things and I can lend further advice. One thing to note: no matter which thing you choose to use music for, check out Adobe Audition's "Remix" feature to customize the length of your backing tracks. It's freakin' genius.
Thanks,
Kevin