Sakai Clan Helmet is a helmet you receive while playing "Ghosts from the Past", from Jin's Journey. It is a story reward, so the tale is the only way to get it.
Play "Ghosts from the Past". The helmet is handed over during the tale, not at a shop.
Sakai Clan Armor
The Sakai Clan Armor is the armor of Jin's father, Lord Kazumasa Sakai, which Jin reclaims at Omi Village during Ghosts from the Past. It boosts melee damage and health and extends your Standoff streak, so it is the set for open fights.
Sakai Mask
Sakai Mask is a mask you get by finishing "Ghosts from the Past", from Jin's Journey. It is a story reward, so playing the tale is the only way to add it.