Soul Stamps are Key Items in Xenoblade Chronicles 2. They can be obtained as rewards for completing the Merc Missions Soul Lessons 1, 2, 3, 4 and 5 during the quest Bearing Her Soul.
Xenoblade Wiki
Soul Stamps are Key Items in Xenoblade Chronicles 2. They can be obtained as rewards for completing the Merc Missions Soul Lessons 1, 2, 3, 4 and 5 during the quest Bearing Her Soul.