Locating rare books materials for use in your 1100 or 1200 level language class is easy. You should prepare a list of items you are interested in, and then consult with Karla Nielsen and Jane Siegel to ensure that they are all in suitable condition.
Early print materials are, fortunately, catalogued in CLIO. The easiest way to find them is to search for the text or author, and then order your results by "Published Earliest."
Manuscripts are not catalogued locally, but they may be found in the excellent Digital Scriptorium.
Using the Advanced Search tool, enter your author or text as a search criterion, and then find "New York, Columbia University, Rare Book and Manuscript Library" in the list that drops down for "Current Location." This will restrict your search to manuscripts held in RBML.
The Digital Scriptorium provides clear and legible images for nearly all MSS, which is an excellent way to gauge what kind of manuscript you will be dealing with.
Your best resource, however, is your curators and librarians. They will know about uncatalogued material, and will have experience with other RBML teaching sessions.