To efficiently farm materials, check nero checkpoints and research sites first since they consistently have gunpowder and kerosene in storage areas. The best spot is around lost lake, where you can create a good loop through abandoned cabins and ambush camps, grabbing cans and alarm clocks as you go. Military trucks usually have decent gunpowder stashes, and most gas stations are reliable for kerosene which you'll need for molotovs

Once you've cleared an ambush camp, make sure to grab the bunker recipes, and remember that materials typically respawn after about three in-game days. A really useful trick is burning freaker nests whenever you can since the residue is great for crafting those bolts that make enemies fight each other. Just keep in mind you'll need to progress through about 7% of the nero storyline to get that recipe first

Always check storage areas in nero sites carefully, sometimes good stuff is hidden behind shelves or in corners. If you're running low on scrap, breaking down unused melee weapons is a quick way to stock up