Out of aluminum? It happens. In the craft-happy world of Fallout 4, you need a lot of it, especially if you're into weapon and armor mods. But if you hit up this location in the northeastern section of the Commonwealth, you will find a treasure trove of trays, giving you enough material to last, oh at least one crafting session, at least.
North of the Museum of Witchcraft and east of the Coastal Cottage settlement is Mahkra Fishery, a fishpacking plant that was in operation when the bombs fell during the Great War. You may find Raiders or Synths stationed here, so come with adequate weapons but enough room in your inventory to cart away your aluminum. On the ground level you will find a lift that will take you into a basement area, where two assembly line areas are rife with trays. You will find about 80. Pack 'em out and store them in one of your Workshop benches at a settlement.
Already looted that place and still need more aluminum? Cans are probably your best option, as they are numerous and do not take up much weight in your inventory. TV dinner trays are also abundant, light, and provide 3 Aluminum each. Arturo in Diamond City and KL-E-0 in Goodneighbor also sell shipments of aluminum in quantites of 50.