In addition to the resource mentioned in post #8, above, one can find the digital version of volumes 14-21 of the
Papers of Dwight David Eisenhower here.
These volumes, covering both of President Eisenhower's administrations, were originally published as a part of Johns Hopkins University Press's twenty-two volume collection of Eisenhower's papers.
ETA: The provided link no longer works. I will update it if I find one that does.