Go to Amazon, do a search on "genere royalty free music downloads" replce genere with the actual genere of music you're looking for. If you wanted rock, you would search "rock royalty free music downloads. You can leave off the downloads and it will give you that as a choice.
It wil list a huge selection and you can even play them to preview (prelisten?)
Hope that helps you pick up a few tracks for your next project.