Where would you get sheet metal for custom road signs?

The signs will be 12" by 18", with rounded corners, painted white, with
black two-inch letters.

Steel would work, but aluminum would be better.
Thickness would be something like 0.060 to 0.080 inches.

I maybe need material for 10 signs, at most.

I'm guessing welding shops will have the raw material at a good price.
Sheet metal shops too.

Ace and Home Depot have the raw plate steel, but the plates themselves cost
as much as signs being made, so the only question here is where to get about
12 feet by 18 feet (roughly) of plate metal inexpensively.