If you just want place-holder/temp music - meaning you won't be using it in your final cut - you could use music from other films, classical music, etc. That will set the general mood you want and give the composer an idea of the direction you want to go. Just don't fall into the trap of falling in love with your temp music.
I'll second SoundDogs as a good place to get tracks. You can buy by-the-cut, and entire CDs range from $30 to $100 with some nice deals on collections. You can also get sound effects the same way. I've used them for both.