Makrah's fishpacking, has something like 93 trays all containing one aluminum, which nets you with 93 aluminum, that would cost you a little over 5000 caps to buy since a shipment of aluminum(25) is around 1500 caps.
I have noticed certain locations that give out crazy loot. The giddy up buttercup factory is a winner but you'll have to explore the map to find it, that's the best part for me.
http://www.gamersheroes.com/game-guides/fallout-4-list-of-vendors-who-sell-settlement-material-shipments/
take a look bro
Vaults always, for the most part, have a nice selection of loot. Everything from food to microscopes. When I am trying to find materials, I usually check out factories, buildings downtown, vaults, and any gas station/outpost you might stumble upon.
Not the best tips I know. Most places will have TONS of specific loot for that area/building.
Best of luck hunting!