No game is truly finished without some background music. So here are a few places you can find it.
Newgrounds has lots of great free music that you can use for your games. It’s going to take you awhile to look through it and find good music, but there is enough there that you’re sure to find something you like. You’ll probably have to give credit to the artist though.
Shockwave Sound has a nice library of sounds and music, but it’s going to cost you quite a bit. Prices range from a few bucks to around $50.
SoundRangers has some good, high quality loops and effects at decent prices. Sound effects are only going to set you back $2, while loops are only about $20-$30.