The Unwomanly Face of War - Vol. 1 Ch. 2

[Colonel-Picquart]

Not coffee, my dear, Ersatz Coffee... Courtesy of the british naval blockade ;p

Actually since this is WW2 the German government made every effort to spare the civilian population such things. There was no naval blockade of Germany in WW2 because of the invasion of Norway which made a blockade impracticable. So consumer items like coffee were available for much longer than you may think.