Music scores are cited in the same way as books.
Handel, G. F. Trio Sonata No. 1. Arranged for 2 alto saxophones by Himie Voxman and Richard Hervig. 1733. San Antonio, TX: Southern Music, 1989. Print.
Thomas, Augusta Read. Incantation: For Solo Viola. G. Schirmer, 2006.
Beethoven, Ludwig van. String Quartet in B Flat Major Op. 18 No. 6. Universal Edition, edited by E. F. Kalmus, 1937. IMSLP, imslp.org/wiki/Special:ReverseLookup/90199.
Schubert, Franz. “Gefrorne Träanen.” Winterreise, Op. 89 (High Voice), by Franz Schubert, Urtext of the New Schubert Edition, edited by Walther Durr, Bärenreiter, 2009, pp. 8–9.
Unpublished scores are treated the same as unpublished books.
Bach, Johann Sebastion. Cantatas. Selections. Seven Cantatas by Bach, once owned by Mendelssohn. Copyist's manuscript. [1700-1750]. Department of Music Manuscripts and Books, Pierpont Morgan Library. https://ww.themorgan.org/music/manuscript/114166.