If you have another critical reader take a look at your manuscript, you can often fix some big-picture problems on your own, which allows the editor to spend more time fine-tuning and focusing on smaller (but still important) issues and areas to polish. Essentially, getting as many eyes on your manuscript as possible will allow you to gauge most effectively how a wide range of readers will respond to your story.
A professional editor is often the last step toward ensuring your story is at its best before putting it into the hands of readers at large. But if you have a critique partner, who can help you spot holes in the narrative or point out unconvincing character motivation, you can present a more complete version of your manuscript to your editor. This, in turn, will make the editing process more efficient (and cost less money, in the long run), because the editor will be able to tighten up the manuscript much more smoothly without working through major rewrites.
I have found my best resources for beta readers are through the friends I have met at local writing conventions. Often, writing workshops will include a roundtable critique of your first chapter (or short story), and in reading the work of others and hearing their feedback on your own writing, you can gauge who might be a good fit for a critique partner. The one thing to keep in mind is that some writers read a story and see it playing out their way instead of your way. This feedback will be easy to spot and must be taken with a grain of salt. No one knows your story better than you, and if the feedback you are getting tries to take your story in an entirely different direction, it is completely OK to disregard it--just keep in mind that the reader may have misinterpreted your intent because it is not clear enough in the manuscript, and that can be more productive to focus on.
Having a few beta readers can also help to give you the thick skin that is so essential for every writer. Working with an editor for the first time can be difficult. While I try to make every writer understand what I like in the manuscript and what is working, it can be hard to see the praise through all the red "ink." Finding an understanding that feedback is something you can use to improve your story (or something you can ignore for the sake of your story) is a critical tool for all writers, and this is an area where beta readers can also be very helpful.