Look through lyric databases to find the exact name of the song for which you want to obtain sheet music. Sites like LeosLyrics.com and AbsoluteLyrics.com have a wide range of lyrics. If the music is purely instrumental, you will obviously be unable to search for the title by utilizing a lyric-based search---instead, call a local radio station to ask about a recently played song, browse music CDs at the local library or listen to clips on Rhapsody.com until you find the right song.
Search online sheet music databases for free downloads of popular scores. Music-Scores.com has a wide range of classical scores.
Consult tablature alternatives if you are unable to find free sheet music. Tablature is less precise than traditional sheet music, but it is easy to read for even novice musicians, and free tablature is often easier to locate than free sheet music. MXTabs.net is a good starting point because MXTabs' archives are reportedly obtained legally, making the site more credible and accurate than tablature sites featuring unlicensed tablature interpretations.
Consider purchasing sheet music if you're unable to track down free copies. Band websites sometimes sell sheet music directly to consumers. Purchasing directly from a band or record label ensures quality and score accuracy.
Browse local music stores. Music stores often stock a wide range of sheet music, from contemporary to classic. If you cannot find the desired score on the store's shelves, ask an employee if special ordering is possible.