When you choose to upgrade a building, a list of materials required to upgrade it will become available. You can select the icon of a material to get a brief description of where it can be found. Fixing potholes, benches, and garbage bins can be a great way to find different materials. At level 15 the Global Market can be unlocked, allowing you to buy materials directly from the dispensaries of other players!

Other times you will see them available as rewards for completing quests, harvesting buds, competing in the Hempire Cup, or completing City Hall Tasks.