Foreign Relations of the United States series. Check here and here for online versions.
Presidential Library webpages. For example, here. Each president has their own. Clinton’s is surprisingly good for 1990s-era material.
Specific U.S. government agency websites. Most important is the CIA’s CREST system. See also here. Other agencies, such as Department of Defense and National Security Agency, have portals as well. NRO’s is here. In general, search Google for the department/agency name and “FOIA” or “electronic reading room.”
Private for-fee collections/databases. Our library system has access to only some of these (check here). If not, you may be able to access them through other unversities or colleges, including Chicago-area locations like Northwestern. Digital National Security Archive. Declassified Document Reference System (now called Gale Declassified Documents Online). Archives Unbound. Proquest History Vault. BrillOnline Primary Sources. Hathi Trust Digital Library.