Finding music by Medieval and Renaissance composers can be tricky, but there are many resources at your disposal.
First: Always try searching the catalog. More and more catalog records are being updated to include complete contents. Use the "Resource Type" limiter on the the left-hand side to narrow your search to scores, books, or CDs.
Know though that many of the complete sets have yet to be indexed, and even if they have, the older languages can present multiple spelling variants that can derail a search.
Second: If you are looking for a specific composer, consult Grove - the work list will usually start with a bibliography of editions and then list each work with a locator reference.
Third: Search the Index to Printed Music - this has replaced older print indicies such as those by Heyer or Hill and Stephens. You can search for individual titles and will find a reference to the volume that contains it. You can then search that title in the library catalog to find the call number.