It’s difficult to answer this question without first addressing what an editor actually does. Editors can review the content of your writing (characterization, pacing, plot, etc.), which is often referred to as content editing; the form of your writing (the grammar, the punctuation, etc.), which is often referred to as copyediting; or both. (By the way, proofreading is indispensable, but it’s the final step of checking for typos and other glitches once the book is ready for print—any writer who thinks that proofreading is editing is in grave danger of getting ripped off.)
EFA has an excellent reputation in the field, and their high annual membership fee keeps out the riff-raff and newbies merely flirting with the profession. For writers, EFA also has a very useful rates survey for the different kinds of editing, helpful for anyone shopping around: (the-efa.org/res/rates.php) For those keen to find a fully vetted editor, try Editcetera (editcetera.com). This group not only tests every applicant, they also supervise for a full year. Of course, their rates are higher. Another highly regarded editorial organization is Bay Area Editors Forum (editorsforum.org), with a searchable database of editors. I’m a member there.… Read more »
Work in house first. That’s the best way to learn all you can about editing and proofreading and make good connections. I have met several people I’ve worked with from hanging out in Twitter and responding to tweets asking for an editor or proofreader, but I have had the most success from business connections I made while working in in house. Other things – a web site, professional associations such as the SFEP and the EFA (though I can’t say much more on these as I am not a member of either yet!), social media.
I'd suggest working through each change so you can learn for next time, rather than just clicking Accept All on the Track Changes version. This will take time, but it's well worth it. I usually accept 80-90% of my editor's changes and the manuscript is improved considerably by the experience. After the line edits are updated, I send the manuscript to a proofreader for the final check before publication.
I would add the suggestion to be prepared for the editor to request a look at the manuscript (not just a short sample) to prepare an accurate estimate. This has been my practice for some time, and it allows me to provide a flat fee for the entire project. This makes budgeting much less complicated for the author, although of course I am willing to bill by the hour if preferred.
I cannot speak for all editors, of course, but I know I and many of us would be open to editing just part of your book at first, say, a particular chapter or two that you sense needs a lot of work. That is often enough to show you mechanical errors that you tend to make repeatedly. You can then go through the entire manuscript yourself looking for as many of the same kind of errors as possible. You will sharpen your basic writing skills in the process, and it will pay off in all future projects.