Resources: dieselpunk and atompunk
A large part of the looks of diesel and atem involve a lot of vintage styles.
Of course, you can very easily get your hands on vintage reproduction, in various price ranges (at least you can before Brexit really takes hold).
So here's a few (feel free to add more!)
The Seamstress of Bloomsbury
The House of Foxy
The Heritage Milliner (also does bespoke hats)
Splendette (fakelite reproduction accessories)
Freddies of Pinewood
That said, there are some really good youtube channels out there that sew garments step by step, for those that prefer to make their own clothes.
I am personally very fond of The Closet Historian, but Gertie's World is also pretty good (especially if you want to figure out how to use Charmed patterns).