How do historians get their hands on diaries and letters of ordinary people?
score:2
Accepted answer
Partial answer:
Regarding letters, there is a collector's market in covers, which means a stamp with the complete envelope and sometimes with the content as well.
Regarding events in living memory, it is bread-and-butter work for historians and their undergraduate and graduate students to collect oral history. At the time they might ask for permission to copy documents. (Anecdotal and hence not really evidence: I once helped a grad student to find a translator for a polish letter among mostly dutch stuff.)
Regarding WWII, quite a lot of German officers published their viewpoint after the war, mostly to point out why someone else was guilty of the genocide and the lost war. Other Germans then published to refute those claims. Not all got as much publicity as Oskar GrΓΆning did, there is lots and lots once you go into non-digital archives.