This resource mostly appears in “wilderness”, i.e. any place in the world that doesn’t have a “street address”: forests, lakes, deserts, mountains.
So whenever you are hiking outside of the city make sure to capture a piece of land to see if any supplies or their sources pop up nearby! Quite often you will find that the resource appears right next to you, so keep checking in even if no resources are seen currently. 


Alternatively, you can scrap items for supplies or build a Refinery using Unobtainium. If you do possess a Refinery then you will always be able to collect Supplies from a base that built it without the need to go hiking.