Depending on your definition of "cheap" - I think the best approach would be to hire someone within your budget. That way you can collaborate with that composer and really get the best end product that fits your needs and vision.
If that's not possible then google "royalty free music" and you should get many hits for libraries. Audio Jungle, Sound Rangers and Sound Dogs come to mind.
I agree, there are plenty of talented, professional & organized composers who are looking for opportunities to collaborate. I'm sure many of them (I am one myself) would be happy to consider a collaboration with you and help each other out.