The best way to find an editor is to go through an editor's association, like the Editors' Association in Canada (http://www.editors.ca/) of which I am a member. The associations are a great place to start because they have a list of members for hire. There's an online directory that is searchable to help you find the right editor for your project. If you aren't sure, you can phone the branch location that applies to you (Toronto, BC etc) and let them know what your project is; the names and contact info for a few editors with skills that match your needs will be given to you. It's up to you to phone and interview each editor to determine the rates and personality that fit your work. Give 1 book or project to 20 different editors and you'll get 20 different edits because editing (substantive and the more involved edits anyway) are subjective.
In the US, I know of the Editorial Freelancers Association (http://www.the-efa.org/).
Google searches are also an excellent way to find freelance editors and writers. Search terms like "freelance editor" with any combination of location and type of edit will yield some results. Check out the editor's website and then contact them even for a general conversation beforehand.