Many early works are quite short and are published in large anthologies. They may also be anonymous, so there's no composer's name to include in your search. The search tips and "best bets" below will help you get started, but please feel free to contact the librarian at firstname.lastname@example.org for help with your specific search.
You'll also need to exercise care in selecting a score. Since music of the Middle Ages and Renaissance was not notated in the form modern musicians are used to, you'll likely be consulting scores that have been transcribed from a different notation system, and editors will have made many decisions about how to represent that music in modern notation. Often, this will mean that they've added details not actually found in the original, and some of these choices are controversial. When consulting scores, be especially careful about older editions, because scholarly consensus about some matters of interpretation has changed.
1. Try a search with the name of the genre and period you're interested in. (Try medieval sequences for example, or renaissance motets). You can add a composer's name as well, if you have one in mind.
2. Use the filters to limit your results to "scores."
3. Some results in the catalog will list the full table of contents and others will not. If you are able to identify one piece in a collection, you can use Interlibrary Loan to request it. I'd suggest also requesting a copy of the introduction, if there is one. You can also use Local or UW Requests to request pick-up of the full volume.
In cases where the piece you want might be located in a larger collection, locating the right pages can be difficult. Because many early works are relatively short, they're often published in collections or anthologies with many other pieces. Looking at tables of contents, reference articles, and catalogues can help you figure out where certain works are available in larger collections. Some helpful starting points are:
1. Grove Music Online entries for composers often have a "works" list that includes the location of each work in collections. At the top of the "Works" section you'll usually find a list of major collections that include the composer's work, with an abbreviation assigned to each one. Then, each piece will be listed with its location in those volumes, as in this example from the entry for Francesco Landini.
2. La Trobe University's Medieval Music Database is a bit outdated, but it lists 14th century works by composer, and gives location in anthologies for each song. Look under the heading "Editions" for modernized scores and the page number where you'll find the specific song.
3. The Corpus Mensurabilis Musicae is a large multivolume collection of early music; the series website lists the contents of each volume. You can browse by composer/anthology to find the volume, and then click to browse the table of contents.
4. Some anthologies, like this one, are included in Google Books, so you may be able to view the Table of Contents there, and use that information to make requests through the library.
UWM subscribes to a few databases of streaming music. Within the databases, you can search for keywords or browse by time period and other criteria. The best bets for early music are: