Once you've exhausted all the usual suspects (MLA Bibliography, JSTOR, and of course the Rush Rhees and Robbins Library stacks), you can turn your attentions to more specific databases. These resources will help you find overview articles, quick facts, and definitions, as well as scholarly articles.
Abbreviationes (electronic database of Latin abbreviations)
Brepols Cross Database Search (UR online; includes the Library of Latin Texts, MGH, Aristoteles Latinus Database, and the Archive of Celtic-Latin Literature)
Encyclopedia of the Middle Ages (UR online)
Feminae (index of secondary sources for the study of medieval women)
Index of Medieval Art (database of images and information about medieval art, architecture, and material culture)
International Medieval Bibliography (UR online)
Iter: Gateway to the Middle Ages and Renaissance (a wide range of secondary and primary sources)
The Labyrinth (thematically organized, with links to further online resources)
Late Medieval English Scribes (catalogue of all scribal hands which appear in the manuscripts of Chaucer, Gower, Trevisa, Langland, and Hoccleve)
Lexikon des Mittelalters (UR online; standard encyclopedia for medieval studies)
Library of Latin Texts, Series A (UR online)
Library of Latin Texts, Series B (UR online)
New Catholic Encyclopedia (UR online)
Orbis Latinus (Latin place names and their modern language equivalents)
Oxford Bibliographies Online (UR online)
These resources feature collections of primary sources online.
Monumenta Germaniae Historica (in German)
Patrologia Latina (UR online)
Dictionary of Old English Web Corpus (UR online)
Early English Books Online (EEBO) (UR online)
Some of the important journals in Medieval Studies. River Campus Libraries holds many of these in both digital and print formats.