Does anyone know where I could find Masamune, the Katana, I just completed the Giant crystal at Giruvegan and got the treaty blade? Where do I go to get Masamune?

Man of Mystery Elite Mark Hunt....the reward for killing the mark, Gilgamesh, is the Masamune

There's also a combination of items you can sell to merchants that makes it available in the Bazaar as a Master Crafted Blade...not sure what items you need to sell though.