A variety of German archival material ended up in Russia after World War II. Henrik Eberle’s Letters to Hitler (Malden, MA: Polity, 2012), based primarily on material in the Moscow archive, is an example. It is an edited and condensed version of a book originally published in Germany in 2007. The book includes a wide range of letters to Hitler.
The book also recommends the German Propaganda Archive (p. 202) as a good site for those wanting further information.