Most of the items in this category are common materials purchasable from a Crafting Supplies Merchant, and are used in many recipes for that craft.
Some are very rare materials used as an Ingredient in certain Rare recipes. Some are found in a Mysterious Bag, some are mob drops in certain Dungeons (such as the Goblin Mines), and others are found only as rare exceptional results while Gathering from resource nodes in zones near the level of the material.