Articles are found both in journals and in collections of essays (such as the papers presented at a conference, and later edited for publication). the bibliographic databases presented here generally index both journals and collections of essays. Academic Search Premier, which is a general, interdisciplinary database, even indexes popular magazines, although one can limit a search to peer-reviewed journals. The first two resources here are specific to the Middle Ages, while the others index articles in specific disciplines.
You can also search for articles in the search box on the Library's main web page, although you will only find articles that the Library has in digital form.