One of the things I learned working for an educational publisher was that the publisher will send out manuscripts for review to people they consider experts.
The department I work in develops products for educator professional development. When a manuscript is in pretty good shape, a supervising editor will contact people to see if they’d be interested in reviewing the manuscript and providing specific feedback. This is a paid review. If they agree, the manuscript goes out to them with instructions on what kind of feedback is needed and a deadline.
Since we work with professional development, the department keeps an eye out for potential reviewers anytime they go to a conference or hear about a good workshop that someone else attended.
That’s not the only way publishers find reviewers, though. Free Spirit Publishing posts their need for manuscript reviewers on their employment page. They also post freelance information. I included them here today because they specifically state the need for educators in the Manuscript Reviewers section.
UPDATE: The call for manuscript reviewers has been removed from the employment page. If anyone was able to talk with them, perhaps you can let us know what happened. - Laura