Bowman's Straw Hat is a straw hat favoured by archers. You get it by finishing "The Art of Seeing", the second of two Yuriko side tales.
Finish Yuriko's first tale, then complete "The Art of Seeing". The hat comes with the reward.
Charm of Yuriko's Keepsake
The Charm of Yuriko's Keepsake is a minor Ranged charm in Ghost of Tsushima. Each arrow has a 15% chance to poison its target. You get it by finishing The Proud Do Not Endure or The Art of Seeing.