Often, Spiritfarer will indicate that you need a specific resource to add improvements or buildings to your boat, or to progress past a certain barrier.
In the most general sense, the way to finding new resources in Spiritfarer is to keep exploring, and keep fulfilling your spirits’ requests. They will lead you to new areas and new resources.
In a more specific (and slightly more spoilerish) sense, you will sometimes need to release one or more spirits into the afterlife before you are able to progress into new areas. We know it can be difficult to let some spirits leave, but it is, after all, the spiritfarer’s duty.