Midnight Shark is a dye for the Half Bow. It is a reward for completing "Laid to Rest" (Ishikawa's tales, part 9 of 9). Like every dye it only changes colours.
Play "Laid to Rest" to the end; the dye comes with its rewards. It cannot be bought.
Once unlocked it shows up as a colour option for the Half Bow.
Charm of Precision
The Charm of Precision is a minor Ranged charm in Ghost of Tsushima. Your arrows deal a moderate amount of extra damage. You get it by finishing The Past Never Passes, Dreams of Conquest or Laid to Rest.
Shattered Mask of Tomoe
Shattered Mask of Tomoe is a mask you get by finishing "Laid to Rest", from Ishikawa's tales, part 9 of 9. It is a story reward, so playing the tale is the only way to add it.