The harmonica is a rare instrument, similar to the guitar, featured in S.T.A.L.K.E.R.: Shadow of Chernobyl and S.T.A.L.K.E.R.: Call of Pripyat.
In Shadow of Chernobyl, one can be found in the construction site next to the transition into X-18 in the Dark Valley. It can also be looted from stalkers, but it is extremely rare.
In Call of Pripyat, a guitar, harmonica, and a blank PDA can be purchased from a stalker.