Additionally, I think that many GREAT, very reputable editors absolutely will not complete a sample edit for free. They can point you to their work, their testimonials, and their references, though, free of charge. And if you’re curious why editors I know do not offer a free sample edit, check out this video to illustrate the point. https://youtu.be/essNmNOrQto
For my book, Career Change, I gave an early copy to people working in my department in the day job who I knew were dissatisfied with what they were doing. They came back with questions and suggestions for what to include as additional material. For The Healthy Writer, we asked medical doctors to read it as a sense check (even though my co-writer, Euan Lawson, is a doctor!)
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.
It’s always sensible to follow due diligence in submitting your first draft of your book to an editor by making as fool-proof a copy as possible for you because first, it is your own creation, and secondly, a horrid draft sent your editor can end up frustrating both you and him or her. Make your own corrections to the extent you can, so that will make the job of your editor that much lighter and the end result would be a lot more effective.