The Papers of Archibald MacLeish are at the Library of Congress,

http://www.loc.gov/rr/mss/text/macleish.html

The Archibald MacLeish Collection and the Archibald McLeish Collection Addition are at Yale University,

http://webtext.library.yale.edu/xml2html/beinecke.MACLEISH.con.html

and

http://webtext.library.yale.edu/xml2html/beinecke.macleishadd.con.html