Potion Crafting
To start with, a character needs a formula for a potion in order to brew a potion. The formula is like a recipe. It lists the materials needed and steps required to make the item. An item invariably requires an exotic material to complete it. This material can range from the skin of a yeti to a vial of water taken from a whirlpool on the Elemental Plane of Water. Finding that material should take place as part of an adventure.
In addition to facing a specific creature, creating a potion comes with a gold piece cost covering other materials, tools, and so on, based on the potion’s rarity. Those values, as well as the time a character needs to work in order to complete the potion.