Professor Jeremy Popkin, the son of UCSD Philosophy Professor (and Department Founder; 1963-1973) Richard Popkin, has informed me that his father's papers are now available at the William Andrews Clark Memorial Library at UCLA.
Jeremy Popkin, as I pointed out before, has authored a biographical essay about his father entitled "In his own words: Richard Popkin's career in philosophy", pp. 259-293 of The Legacies of Richard H. Popkin (Dordrecht, 2008). This is volume 198 of the International Archive of the History of Ideas, a book series established by Richard Popkin and Paul Dibon (visiting UCSD Professor 1964-1966). Jeremy Popkin tells me that he is now editing an interview of his father about his relationship to Herbert Marcuse. Should be very interesting-- I'll try to keep you posted.