On line or off , the best place to start is Olin Library. It always helps to ask a reference librarian for advice. The library has a page specializing in medieval studies reference resources.
Medieval Sourcebook--Full Texts
Medieval Sourcebook--Selected Texts, which is the basis of the on-line reader.
Sources for the History of Medieval Religion and the Church
Labyrinth, an on-line medieval studies resource that includes links to bibliographies, texts, and some on-line journals.
A second advantage to journal articles is that they may represent the only scholarly work done on a topic, which hasn't yet warranted a book, especially a book in English. I find also that an author often first presents his or her results in an article that is more pithy, if not nuanced, than a longer book treatment would be. Journal articles are also the places where most scholarly debates are worked out and can act as catalysts for your own further questioning. The most relevant journals vary greatly depending on your subject. While you might find what you need by browsing, the many dense-looking volumes can be repellent or overwhelming. The best way to find an article is to examine the bibliographies and footnotes of people who have written on the subject. Some online bibliographies can help, as can a flesh-and-blood librarian, but I advise talking to me and getting a recent book on the subject and checking the back. A good scholarly book will pretty well list all the relevant works known. Footnotes can similarly guide you to other good articles. When you find your interest piqued, look at the nearest footnotes. They will often indicate where the author went to learn more, and you can follow. Below are a variety of journals and bibiliographic aids relevant to this course.
Relevant Focussed Journals
Notes and Citations in Papers
See Paul Halsall's advice on citing
electronic sources.