Ghost Mask is a mask you get as soon as you start "From the Darkness", from Jin's Journey. It is a story reward, so playing the tale is the only way to add it.
Start "From the Darkness" in the main story and the mask is handed to you early in the tale.
Ghost Armor
The Ghost Armor is the dark stealth outfit Taka made and Yuna gives Jin during From the Darkness. It slows enemy detection, gets you into Ghost Stance faster and makes kills scare nearby enemies, so it fits a hit-and-vanish style.
Ghost Headband
Ghost Headband is a headband you get from "From the Darkness", a main story tale in Jin's Journey. There is nothing to search for: play the tale and it arrives with the rewards.