Complete listings of composers' works, including some basic information on the primary sources, can be found in Oxford Music Online (available on the Library's homepage, under "Databases").
To look up a composer's worklist in Oxford Music Online:
For each work, you can expect to see most or all of the following data:
Work number (opus, thematic, etc.)
Title
Key
Date of composition
Date of first performance
Publication date of the first edition
Remarks (revisions, dedicatee, etc.)
Volume in the composer's collected edition(s) in which the work appears
More information about a given work may be found in the main text of the article, and in the bibliography at the end of the article.
Once you've gathered this initial data from Oxford Music Online, you may want to consult the composer's thematic catalog for more detailed information on the work's primary sources.
Identiying the primary sources of medieval works often requires a different approach, because fewer sources survive from this period. For most works, we no longer have the composer's original manuscript -- so our principle sources are copyists' manuscripts, which were typically gathered into collections for use in the cathedrals or by private patrons.
In Oxford Music Online, you can find descriptions of the best-known of these collections. To see a list of them:
You can also look up individual composers' biographical entries, for more information on the sources in which their works appear. You'll see hyperlinks that identify the libraries that own these manuscripts.
Next, you may want to consult RISM Series B for more detailed information on a work's primary sources. See the indexes for titles and composers at the back of each volume.