Many people recommended ALRECO in Henderson, Colorado (near Brighton) as a source for aluminum. I haven’t been there, but I’m sure they would have been able to satisfy my needs. However, before I found out about them, I discovered that one can buy a 12 inch by 24 inch sheet of 1/8 inch aluminum from Amazon, and that it’s even eligibile for Amazon Prime shipping. It’s actually a product sold by Small Parts Inc., but they are using Amazon for fulfillment.
At $52, it’s a lot more expensive than other alternatives. However, for my situation it was ideal. I’m in Colorado, but I needed the aluminum to be in Oregon. I was able to get it there overnight for $56. If I’d purchased it locally and shipped it myself, it would have cost more than that.