Official Sources
FIPS Country Codes: http://geonames.nga.mil/ggmagaz/geonames4.asp and  http://www.state.gov/s/inr/rls/4250.htm
ISO2 Country Codes: http://www.iso.org/iso/country_codes/iso_3166_code_lists/english_country_names_and_code_elements.htm and http://www.iso.org/iso/english_country_names_and_code_elements#s
ISO 3 Country Codes: http://unstats.un.org/unsd/methods/m49/m49alpha.htm
GENC2 and GENC3 Country Codes (Geopolitical Entities, Names and Codes): Issued by the National Geospatial-Intelligence Agency https://www1.nga.mil/Pages/default.aspx