Aside from published books, biographical information on popular authors can be found in magazines and newspapers. When a new book comes out, the book reviews will generally appear in the book sections of the local newspaper and magazines, rather than in academic journals. Often these reviews will include some biographical tidbits, but you may have to look through several articles to piece together the more complete information you would find in a literary encyclopedia or dictionary on a literary author.
Most well-known popular authors will show up in scholarly journal databases such as JSTOR and Gale Literature. If you're still having no luck, try looking in popular magazine and newspaper databases to find biographical articles on lesser-known popular authors. Some other databases to try include: