I saw some of these on my trip to the Washington National Cathedral on May 19, 2015 on display. These were letters from WWII service people as well as others like an artist who sketched wounded warriors in a Veterans' Hospital in Philadelphia in WWII.
War Letters Project | Chapman University
http://www.chapman.edu/research-and-...awl/index.aspx
There seem to be a lot of institutions and/or organizations that collect war letters.
http://www.war-letters.com/