Warrior's Bounty is a dye for Samurai Clan Armor. It is a reward for completing "The Traitor" (Masako's tales, part 4 of 9). Like every dye it only changes colours.
Play "The Traitor" to the end; the dye comes with its rewards. It cannot be bought.
Once unlocked it shows up as a colour option for Samurai Clan Armor, so you need that outfit to use it.