Where can I find the Marumasa?

To be more specific, dungeons are on either edge of the map. If you reach the ocean, turn around and try the other side. Dungeons have special brick walls that are unique. It may have, for instance, green or pink bricks. This is the only natural occurrence of them in the game (aside from floating islands and underground jungle shrines, which you aren't going to run across on the surface) so it's pretty obvious. Impossible to miss.

You then need to kill Skeletron, a potentially very challenging boss if you don't have good armor (much harder than eater of worlds of eye of cthulu). To summon him, visit the old man at night. I won't provide a strategy here, but he's pretty tough. Be ready for a hard fight, and if you (and all your friends) die, he will despawn. Once you kill him, you may enter the dungeon as you please. Or, if you're gutsy, run in while he's distracted by your friends. (trying to enter while he's an old man will result in you dying instantly)

(TL;DR) Muramasa has a chance of being in any chest in the dungeon.