The Big List Of Royalty Free Music And Sounds (Commercial Edition)
Sites where you can buy royalty free sounds and music.
For custom music check out The Big List Of Musicians and Places To Find Musicians.
Consider also to check out The Big List Of Royalty Free Music And Sounds (Free Edition).
In case you want to license a “pop song” for your trailer consider to read this interesting article called How to License a Song for your Indie Game Trailer (Hint: It’s very unlikely that you can afford it).