Shining Resonance Refrain

Materials are found from glowing harvest points and given by allies during conversations with positive interactions, but most will come from battle. There are tons of different materials types in the game, so this will come with no trouble along the way to completing the story. You don't have to have 200 unique ones in your inventory at once, so feel free to use anything you collect as needed. It helps to have the "Steal Attack" aspect on the character you control in battle as well, you'll get a lot of materials this way.