Honest Feedback = Better Writing
All writers know how terrifying it is to hand your work to someone who is going to read it and, for better or worse, tell you honestly what they think of it. Beginning writers make the mistake of asking a family member or close friend for their opinion, and the results are typically "It was great! Loved it!" or "It was okay. Thanks for letting me read it." The writer has no idea if the reader is being honest or trying to be nice. Such "feedback" is useless, when the writer wants to know how the plot flows, if the characters resonate, if there is too much telling and not enough showing, or the setting is inadequately, or overly, described. In other words, constructive criticism.
How does one find a good Beta reader? If you are a member of a writer's group, ask members for suggestions as to whom you might approach. Not a member of a writer's group? Perhaps you know a college instructor who teaches literature. The ideal reader is someone who knows about novel structure and is a good writer.
Still stuck? Check out an online writer's group such as "Women Writers, Women's Books" on Facebook, and inquire if anyone is interested in helping you out.
Your reader should be interested in and experienced with your genre. Having a Young Adult Fantasy devotee critique Literary Fiction or Erotica is unlikely to produce useful results, for example.
And then there's the thorny question of compensation. Unless the reader provides the service professionally, you should not be paying on an hourly basis. However, an honorarium, a meal out, or some kind of quid pro quo is not amiss, particularly if the reader provides detailed, helpful feedback.
If reader feedback doesn't lead to an improvement in your book, assuming you follow at least some of her or his advice, you have the wrong reader.