Make it publicly known what styles of music you are looking for, and open up the position to the public. Let people compose a piece of music for you in the proper formats, and you can feel free to choose whichever composer you think would fit your project best. You can make it known up front what you expect, in terms of song length, quality and pricing, or you can work with whichever composer you choose to discuss these issues.
I'll gladly put together a sample song in whatever style or genre you like, and put it to you for consideration, and so would many others.
As far as pricing goes, in the professional circuit, I believe the authors put forward how much they expect per minute of finished music. As with all things, it can be decided any way you like however.

[edited by - krikkit on March 31, 2002 5:20:23 PM]

For the most part, no, mod galleries such as modplug.com are not free for the picking, the authors of the songs hold all copyrights on their tunes. You need the permission of the author to use it in any venture unless it is stated otherwise within the MOD file.
And to the other question, sure, why not? State what genre, style or "feel" you are looking for, and let the prospective authors audition. I''m sure some other people can recommend other forums where you can ask the same thing.