Free sheet music (piano) for the Frozen movie theme "Let it Go" is available as of 2015 from several sources, of which http://pianomusicsheet.net/let-it-go-frozen-piano/, http://www.everyonepiano.com/Stave-3402-1-Let-It-Go-Easy-Version-Frozen-Theme-Stave-Preview-1.html and http://piano-sheets-for-free.blogspot.com/2014/01/frozen-let-it-go-digital-piano-sheets.html are likely the most dependable.
Of the three sources provided, pianomusicsheet.net provides the most variety of versions of the song. It provides downloadable .pdf files as well as listenable MIDI files. The .pdf files available at pianomusicsheet.net include a solo version and a movie version as well as beginner, intermediate and advanced versions. Pinterest also provides a solo piano version at https://www.pinterest.com/pin/230035493441055677/. It should be noted that none of these sheet music versions provide lyrics. Lyrics can be found at any lyric search website including http://www.metrolyrics.com/let-it-go-lyrics-idina-menzel.html.