This can be found fairly early into the game. After rescuing the blacksmith from the slime caverns, travel one screen to the right of the Inn. The beginnings of a farm will be there, and a box of green fruit... try eating one.
The fruit should stay there for quite some time. I don't recall when exactly the cut-off point is. Try looking there now, and if not, try going back to an earlier save. You can unlock gallery chest with earlier save files!