Where to Find a Composer
Composers are all around on the internet. Note that they range from beginners who might score your game for free to professionals who might charge up to $300/minute or more (note that the quality is likely very different)! You might even luck out and have a friend who is into music composition who can help.
One of the most important things is making sure the person you find has the right setup for what you need done. Someone who just records themself playing an upright piano in their parents' living room with a microphone or writes music by hand and has no one to play it back is not the right kind of composer for what you need (most likely). It is important that the composer you hire has:
1. Virtual/Physical playback available- some method of making what they write audible and putting it in the form of an audio file you can import into your game.
2. A program in which they can create audio loops- when you need looping tracks, they will need to have a program where they can trim or automatically configure the audio file to loop cleanly.
3. A basic understanding of audio file-types and file sizes- knowing which files are the right files for your game is very important to getting the best sound with the smallest weight.
4. A good sense of composition- let's face it... you can't hand your dog a keyboard and ask him to score your game. Make sure the guy/gal scoring your game knows what they are doing BEFORE you hire them. Check out their audio. Make sure it fits with the whole "picture frame" analogy above.
Some good places to start looking for composers:
Scirra Forums Help Wanted section
Newgrounds Audio Forum and NG Collabinator
Some good places to look for pre-made music:
Kevin MacLeod's Website
Newgrounds Audio Portal