For an arts and crafts project, I am looking for on line stores that
sell sheets of metal, of different finishes, dimensions, and thickness,
cut according to specifications, and that provide delivery in the US.

Since no researcher has found your answer in two days, it's likley not
available on the Internet. Have you tried calling sheet metal
shops/fabricators in your area? If you can get one of them interested
in your project, they may be willing to help out with a small job such
as yours, and those guys can make darn near anything.