My Dad was in the Pacific war and fought in many campaigns. How can I find someone who may have known him?

I received my Dad's military records and found that he was deployed in a number of major battles WW2. Guadal C, Cape Glouchester, Pevuvu, Peleilu and Okinawa. His story followed the story from the HBO series, "The Pacific", including Melbourne Au., where he seemed to have married an Australian girl that none of us ever knew about. At the end of his life he seemed sad and angry all the time. Maybe this had something to do with it. How do I find some one who may have known him?