If you're a whiz at GarageBand or can record your own music, I say "go for it" - and email me so I can hire you! But, if you're like me and don't have a musical bone in your body, you can find royalty free music online.
One of my favorite (and free) sites is incompetech. You can browse my genre (horror, funk, holiday, jazz, etc) or feel (eerie, epic, somber, mysterious, mystical, etc). Some of the music is really amazing! Free Sountrack Music is another free site that allows you to search by genre and emotion.
If you can't find anything on these sites - and are willing to pay for your music - you can purchase tracks at Royalty Free Music, Beat Suite, and Stock Music.
For an example, here's the music I used for my Shoreline Trailer!