Hello James,
This morning, I've had a word with my Publisher Wingleader on this. He told me that the Nachtjagd Combat Archive series is available in the USA with:
http://www.aberdeenbookstore.com/Home.htm
I've checked the site and it appears that they have the 1943 volumes currently in stock, you can ask them when they'll have the Early Years volumes in the series in stock too.
Cheers, Theo