The Ronin Attire is a light straw-hat outfit Ryuzo gives Jin during Blood on the Grass. It mixes quieter movement with stronger sword hits, and it shines if you like to stalk from pampas grass before you strike.
Play the main story until Blood on the Grass. Ryuzo hands it over during that tale, together with the Weathered Straw Hat.
Merchants in Izuhara, Toyotama and Kamiagata sell most of the dyes for 10 flowers each, and the White Dye Merchant has Moonlight Slayer. Storm Walker is the default. The dyes match outfits worn by other Straw Hats around the island.