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.
BEA was formed in 1998 to help writers find professional book editors and proofreaders with experience working for traditional publishing houses. It continues to be a matchmaking service for writers and editors and now includes editors who can assist with the self-publishing process. The network includes award-winning editors and ghostwriters who have worked on best-sellers. Writers work directly with the editors they select. Rates vary by editor. Use the submission form for direct contact and a price quote.
Another place to find professional editing service is the team at Ebook Launch. They have a team of professional editors who can polish your book for an affordable price. You can even get a free sample edit of 750 words to help you decide if they're a good fit. The small team at Ebook Launch also provides amazing book cover designs and formatting in addition to their editing service.
I’m sorry you feel that way about my participation in the discussion and defending editors of similar background to myself from what I considered unwarranted disparagement. Perhaps the only approach that would have 100% proof against accusations of “trolling” would have been to allow the claims to stand unchallenged, but that did not seem fair to people who come here to get useful information.
Of course, my c.v. does include traditionally published books, but in those cases, I was hired (yes, on a freelance basis, which you would not “count”) by the publisher rather than by the author. The reason my marketing to authors instead emphasizes my experience with self-published and even vanity press authors is because working with a publisher is different from working directly with an author, particularly with an author who has made a conscious choice not to be traditionally published. Some might even say that working within a publishing house provides less preparation for working with self-published authors than does working with other self-published authors. I, however, would not go so far as to say that, because I really do believe we can come to our profession by different paths, just as people can come to writing by different paths.
Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.