There is no traditional costume for American. But I bet it would be the traditional coat and pant. Or you would call it the suit, that has become really popular for American's these days and other countries copying the western image also wear the american suit accompanied by the tie.
If you wanted a young american costume, probably jeans, a beseball cap and a white t-shirt. For an old american, bermuda shorts, baseball cap, floral shirt, black socks and white sneakers.
