Aluminum can be depleted rather quickly, as many repairs/mods for power armor requires it.
If you mark aluminum, as explained in this answer, items that provide aluminum when scrapped will have a magnifying glass icon next to it when aiming at it to grab. The
Scrapper 2 perk takes this a step further, and will highlight items in the world that provide any materials you've marked.
As for obtaining larger quantities of Aluminum, Shipments can always be purchased for them. Arturo in Diamond City sells a shipment of 50, and Rufus Rubins in Goodneighbor has a shipment of 25. If you have any steady source of income, such as collecting excess Purified Water from settlements, this can be a feasible method to keep stocked.
A few locations in the Commonwealth also have a good quantity of items that will break down into aluminum. Typically, industrial places such as factories and packing plants are a good bet, as are any places that might have stockpiled food (aluminum cans). Per the Fallout Wikia page on aluminum, Mahkra Fishpacking, Corvega Assembly Plant, and the Federal Ration Stockpile are good places to hunt some down.
If aluminum is still in short supply after some purchasing/scavenging, you can check out this answer, which details a set of power armor pieces that needs no aluminum to be fully repaired.