Everyone has a resource that is scarce for them. For some it is Screws, others cannot find Adhesive and some struggle to find Aluminium. Here are some tips that should see your aluminium stockpile sky-rocket! There may be minor spoilers below in regards to place names.
Tv Trays, Aluminium Cans, Trays (not Cafeteria Trays). These items are very plentiful in almost every location and can be scrapped for several Aluminium apiece. Make sure you loot everything you come across and scrap it later. You should always take everything (use fast travel to sell/scrap things if you are encumbered).
Mahkra Fish Factory
Check the screenshot below and you will see a lot of Aluminium Trays just waiting to be collected. This is in the Basement of the factory and there are several conveyor belts of it. There are quite a few robots down here so be prepared to fight for your rightful Aluminium! The best part is that after 3 ingame days you can return and find everything again. I believe someone counted that there are 81 trays to be collected each time to go theremaking it by far the best place to go to find aluminium.
A lot of the items within The Institute have a lot of Aluminium. I found I had a lot after I cleared through this area. You will encounter this naturally during the main story missions.
If you know any other locations or ways to obtain Aluminium then comment below!