Finding Aluminium is so hard!

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.

In the beginning, your best source of aluminium will be aluminium cans. They're not only common, they also give you 2 units of aluminium and have a weight of 0.1; they yield 20 units of aluminium per weight. The next best items are the cake pan, the surgical tray, and the TV dinner tray, which yield 6 units per weight (3 units with a weight of 0.5) and are less common.

However, better than searching the wastelands for cans, you can get lots of aluminium from scrapping metal armor and weapons, if you've got the Scrapper perk. Although they yield much less aluminium per weight than the cans or the trays, they are exceedingly more common and easier to find; half the wastelands is armed and wants you dead, after all. Why look for the aluminium, if the aluminium's looking for you?

Also, don't forget to scrap any trash lying around in your neighborhood (highlighted in yellow in the workshop interface). Cars are fairly rich in aluminium and have no weight (or rather, they weigh so much, you can't carry them in the first place).