If you know which executive branch agency has the documents you want try searching on the web for the name of the agency and FOIA and Reading Room.
For example, a search for <FBI FOIA Reading Room> will bring you to The Vault, a place where the FBI makes some of their records that were requested through the Freedom of Information Act Request available.
Foreign Relations of the United States records major U.S. foreign policy decisions and diplomatic activity. It contains declassified and unclassified documents from Presidential Libraries, State Department, Defense Department, National Security Council, CIA and others. This series tends to publish about 30-40 years after the events.