While writers' magazines and directories have lists and ads for professional editors, there is another potential source of high-quality editing that may be available for less money, and the editors may be available to do your work much sooner.
Check with some journalism departments and college newspapers -- perhaps where you went to school -- and chances are you'll be able to find several bright and eager candidates. Read some samples of their work. Maybe submit a sample chapter for editing. Ask a faculty member for opinions. Then make the deal.
Skill levels will vary, of course, and so will needs and costs. You can pay per hour or per project. Expect to pay more if you need major rewriting than just copy editing.
A student who has a part-time job making minimum wage flipping burgers will probably be thrilled to earn $20 per hour, or $300 - $500 for a project. As a comparison, one publishing company that caters to self-publishing authors recently charged $50 per hour or 1.4 cents per word.
If the job goes well, be sure to put your editor's name in the book, and send a note to his or her faculty advisor.
As long as you're investigating colleges, consider hiring a professor, not just a student. If you're writing in a specialized field, it could be worthwhile to hire a faculty member to check your facts, and pay someone else to polish your prose.