Finding scores seems easy, right? Just type in the composer's name, the title of the work and that should be it, right? Ummmm, not so.
With piano music, we often deal with similar titles (sonatas, concertos, etudes, etc.) and items in collections, whether all of the same genre (sonatas) or a collection of the composer's works for a particular instrumentation.
What to do? Think both narrowly (composer's last name, edition when necessary, work number) and broadly (genre) -- for example a search for beethoven 57 sonatas (then limited to format scores) brings up quite a few scores. If a specific search isn't successful or doesn't find a specific edition, try searching for the composer's last name and genre -- for example chopin etudes -- the search results include collections of all of Chopin's etudes, plus any published separately.