Databases such as CINAHL, MEDLINE, PubMed, and Academic Search Complete are the best places to locate peer-reviewed articles.
Many databases provide a limiter or check-off box that allows you to limit your results to peer-reviewed journals. Be careful, however! Not all articles within a peer-reviewed journal are peer-reviewed articles. Editorials, commentaries, essays, letters to the editor, and book reviews are often found in peer-reviewed journals. These are not peer-reviewed.