Try this website, it's very useful:
It contains addresses for 22,000 embassies around the world (I have it in an excel file actually...).
Here's another from Travel.State.Gov
REGISTRATION / EMBASSY LOCATION: Although there is no U.S. Embassy in São Tomé and Príncipe, the U.S. Embassy in Libreville, Gabon is also accredited to São Tomé and Príncipe and can provide assistance to U.S. citizens there. All Americans in São Tomé and Príncipe are encouraged to register with the U.S. Embassy in Gabon through the State Department’s travel registration website, https://travelregistration.state.gov. Americans without Internet access may register directly with the Embassy. By registering, American citizens make it easier for the Embassy to contact them in case of emergency. The Embassy in Gabon is located on the Boulevard de la Mer; the mailing address is B.P. 4000, Libreville, Gabon; telephone numbers: (241) 76-20-03 or (241) 76-20-04; the fax numbers are (241) 74-55-07 or (241) 76-88-49; the web site is http://usembassy.state.gov/libreville/.
- Chris Smith
How do I get this excel file?
Please contact by clicking on the contact link above or help center.
This link is very good, tells both the embassies in a country and the embassies of a country around the world, with some work.