From Gumberg Library's homepage, type in your search (e.g., choral music) and click on the Search button:
On the left side of the Search Results page, scroll down to "Limit by Source Type", click on Show More, and select Music Scores. Then click the Update button:
Use opus or thematic catalog numbers (like BWV 232 and K.551) to narrow your search.
When looking for compositions with a generic title (sonata, symphony, concerto, trio, etc.), you can truncate your search or add a wildcard character.