1. Search online. Retailers make gowns that are based on period clothing. If you can afford it, this would be a great source for classic prom gowns.
2. Visit your local fabric store. Find patterns for classic prom dresses or dresses that can be worn to the prom, and make your own. Your gown will not only be made just for you, but one-of-a-kind. If you can't sew, hire someone to make the dress for you.
3. Explore stores that sell vintage clothing. Stores like these specialize in clothing from many eras and clothes from the modern era are in the minority. You should be able to find a wonderful classic prom dress there.
4. Experience the thrill of making your own classic dress from scratch by making your own patterns as well as the gown. Explore costume history books or sites. You will make not only a classic prom dress, but also something that can be passed down from generation to generation.