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Atlasgeo.net: Bulgaria
http://www.atlasgeo.net/flags/anglais/html/Ebulgaria.htm
Animated GIF image.
Flags of the World: Bulgaria
http://flagspot.net/flags/bg.html
National, historical, government, military, and political flags.
Theodora.com: Flag of Bulgaria
http://www.geographic.org/flags/bulgaria_flags.html
Images in several sizes.
World Flag Database - Bulgaria
http://www.flags.net/BULG.htm
Presents country statistics and image of the flag.