Sheet music for "Happy Birthday" is available at Let’s Play Music and Music for Music Teachers. These sites offer different types of sheet music for singers and instrumentalists.
Let’s Play Music, which offers sheet music for piano players, features two different arrangements of "Happy Birthday." One is for children who are just learning to play the piano and need a novice version; the other arrangement has a more advanced arrangement intended for more experienced pianists.
Music for Music Teachers offers multiple types of sheet music to download and print, including a simple piano version and two-hand melody version of "Happy Birthday" available in the keys of D, Eb, F and G.