Mapson is the character in Mother 3 that will give you different maps throughout the game. He is only located in Tazmily, and he doesn't talk much. You find him in many different spots inside of the village. Usually, you will find him with a minor character, as you've already gotten most of the maps in Chapter 4, or you will obtain them in dungeons. You will get one at the beginning of Chapter 7 though.
He can also mark your map in Salsa's case, even when he doesn't know what your trying to do.
To get your map marked, you have to go find him somewhere in town. At the beginning of the game, he will usually be in the Tazmily Town Square, but later, he'll be on the bridge that leads to Wess's old house, which got changed into an Old People's Home.