This is track #42 "Yuuhi ni Mukatte" from the One Punch Man soundtrack. Here is a 20-second extract from the beginning of the piece: https://clyp.it/m3qa2cee.
I don't know where you'd find sheet music for it. It is rare for anime soundtracks to have accompanying sheet music (though it does happen, e.g. Symphogear, K-ON!). Your best bet is likely to hope that an enterprising fan has transcribed the piece.