It depends on where you live as to which place is best.
Generally, the foundries are all set up like clock work and organized into zones around America (the continent, not just the USA). Then they go direct to the big suppliers. You want to buy direct from the big suppliers. Russel Metals would be an example. Metal supermarkets, probably the same thing, I don't know. Buy in full lengths, and get better deals.
Scrap yards are great, but near me they are full of steel, they recycle the aluminum immediately.
The general rule I think is that you look in the yellow pages and make phone calls (or emails) to get quotes from the local suppliers with the big warehouses. You don't shop at places that ship metals online or you will pay through the nose.
I use IMS metals https://www.industrialmetalsupply.co...etals/Aluminum There is one in Riverside,CA
Don