Selecting the right database can be the best way to find relevant sources. You may want to start with a general database, like Academic Search Premier and then move on to a more specialized one like Gale Literary Sources. If you can't seem to find anything on your research topic, try different keywords or another database. Remember, you can always ask for help by using Ask A Librarian.
When your professor requires that you read and cite only peer-reviewed journal articles, you should exclude any book reviews. Reviews may be acceptable, depending on your class. Book reviews may be acceptable if the author or work is very contemporary and little or no literary criticism has been published. Furthermore, some book reviews are quite academic. A new edition of a previously published book may appear (for example, Nella Larson's novel Passing), and this event may lead to brand new reviews of Passing in culturall magazines like The New Yorker and Harper's.
You would also want to read and cite book reviews if your paper is a study of the reception of a book in popular media over the decades.
One clarification on the chart: It's not exactly accurate to say that book reviewers have no formal training in literary criticism. That is an exagerration to make the point that literary criticism almost always appears in scholarly journals.