Here is a fashion design from 1939 that includes both front, back and schematic views. You can find more like this in the New York Public Library digital gallery collection. Just enter your search terms. "Costume", "fashion", "sewing" are some good terms to try out. If you use an image, be sure to credit the collection. When it comes to eras, the entire 20th century is often a single category (rather than by each decade).
In this collection you will find more fashion designs like this, book illustrations, and photos. The Andre fashion illustration collection from the 1930's and 40's is a great style resource for that era. The designs include coats, suits and dresses by the Andre Fashion Studios. Most are from the 1930's. This is a huge collection of 5,415 images (so plan to fall down the rabbit hole with this link!).