Louise's advice is excellent! If you know your foot measurements and generally the style/color that you want, finding 40's shoes shouldn't be that difficult. As Louise suggests, if you post a photo of shoes similar to what you have in mind, likely somebody here would have a pair or know somebody who does. Many of us have stock that is not listed anywhere, and might have just what you need!
If you can provide more information, even if without a photo (style, heel type, etc.), you can post to the "Most Wanted" Thread on this board. You'll probably have luck that way.