Available from stock or within a very short lead time: 

Weathering steel, best-known under the trademark COR-TEN steel, is a group of steel alloys which were developed to obviate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years.

Corten steel is becoming more popular by roll formed product end-users. Its unique look and naturally oxidizing finish make it especially desirable for many architectural projects. Weathering steel, best-known under the trademark corten steel, is a group of steel alloys which were developed to obviate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years. Cor-ten is a trademarked name owned by U.S. Steel. This is not available. What is available is steel in an A606 finish. When someone needs corten on the West Coast they are normally referring to A606. If you want cor-ten steel this will be the best available option.
Weathering steels, such as ASTM A588, A242, A606 and Corten exhibit superior corrosion resistance over regular carbon steel as a result of the development of a protective oxide film on the metals surface which slows down further corrosion.