In general, when looking for a beta reader, keep your eyes open. Some people offer their services to others (like either CWTA,P or myself). Other people might not advertise what they can offer because they don't want to be bogged down in requests, but they are still willing because they want to help their fellow writer grow. And others just review around but don't feel like working with someone on a one-on-one basis. ()
Keep in mind what type of beta reader you want. Some specialize in various aspects of writing. I myself only offer beta services for grammar because that's the only thing I consider myself good at explaining. Others might be able to help you with characters, or plot, or description.
Really, to find a beta, just look for someone with a friendly disposition in their reviews and ask. It's just that simple.