The quickest way to find most scores is to do a keyword search for the composer's last name and one or two keywords from the title of the piece. Use the menu on the right hand side of the screen to limit your results to scores.
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Sometimes the piece you are looking for is not available as a stand-alone score. This is especially true for art songs. To find works in a collection, do a keyword search for the composer's last name and the title of the piece in quotes. Check the table of contents to make sure your piece is included.
Musical works sometimes go by several names or nicknames, which can make them tricky to find in the catalog. For example, Beethoven's fourteenth piano sonata could be published under any of these names: