Volumes for 1961-72, and a few earlier ones, are now available at this State Dept. site. The University of Wisconsin at Madison makes the volumes for 1900 to 1918 available for searching or browsing. The Library has volumes back to 1914 in paper form in the documents stacks: S 1.1: .
A portal to the Library of Congress' international resources, it contains links to authoritative information by country, to special databases, including some with legal and historical documents, and to websites of the various regional reading rooms.