I wrote a few chapters of a book several years ago. How do I find an affordable editor?
The most widely used method of finding a freelancer is by asking around for recommendations from friends and associates. The great advantage of working this way is that you can get some rather credible information about the freelancer.
There are several job boards for finding freelancers, including freelancers, online:
There are also directories of editors provided by professional societies and online communities. For example:
If you have specialist requirements, searching broadly on Google (do look at Adwords results), or on networking sites like Linkedin may work well for you.
The downside of finding an editor this route is that you won't know whether they are any good, although some professional societies, such as the Chartered Institute of Editing and Proofreading, do have substantial accreditation programmes giving weight to these names. Asking for a sample edit is valuable here: the information you gain here is much more useful to you if the editor will edit a small sample of your work than provide a highly selective portfolio of their past work.
Postscript - I should add that I don't use resources such as Upwork to try to find work, because the offers that win tend to be so lowball that I do not consider them to be at sustainable rates for professionals. I advise against using these services unless you have a strategy for ensuring that the winner of the contract will perform adequately for your job requirements. Browsing directories of editors and getting in touch with them seems to me to be a better route. Of the above methods, the route that gets me most business is online search, especially via AdWords.