Books suggested by your professor provide an important starting point. Meet with your professor early about your topic and peruse the items she has placed on Course Reserves

Published bibliographies are helpful tools that may lead you to appropriate primary and secondary sources.

To look for bibliographies in the library catalog, do a keyword search using Subject Heading language relevant to your topic and the word "bibliography" (e.g. middle ages bibliography)

If Vassar does not have a source cited in a bibliography search for it in WorldCat and request it though Interlibrary Loan

Domestic Society in Medieval Europe: a select bibliography/ed. M.M Sheehan

Guide to Sources in Medieval History, R.C. van Caenegem