Foreign Embassies in the U.S.: The address and telephone number of foreign embassies and other diplomatic offices in the U.S. are posted on Project Visa and/or Embassy World, usually with links to the Embassy Web site. Alternatively, you can get the telephone numbers and addresses of most embassies in the U.S. by calling Information or looking in the New York and/or D.C. telephone books. (Tip: If you can't find an embassy, try to get in touch with the country's mission to the U.N., which is almost definitely in New York. They can probably give you the number of the Embassy.)
U.S. Embassies in Foreign Countries: The address, telephone number and other information for U.S. embassies and diplomatic missions abroad are posted by the U.S. State Department. You can also look through a directory of Key Officers of Foreign Service Posts.
Listings of prior Ambassadors and other Chiefs of Mission are available on the State Department's Office of The Historian website and in the Europa World Yearbook.
Foreign Embassies in Foreign Countries: Contact information for foreign embassies and other diplomatic offices located in foreign countries is posted on Project Visa and Embassy World, usually with links to the Embassy Web site. If that doesn't work, (a) use a good search engine or two to locate a Web site with the information you need or (b) use the information provided above to contact an embassy, mission or other office of the foreign country in the U.S., then call and ask if they can provide the information you need.