Space Agency Flags
This is a collection of the logos from the national space agencies of; Australia, Brazil, Canada, China, Ecuador, European Union, France, Germany, India, Iran, Italy, Japan, Mexico, North Korea, Pakistan, Romania, Russian, South Africa, UAE, UK & USA. As a bonus there is also flags from The British Interplanetary Society, The Planetary Society and the Mars Society.
Simply copy whichever flags you wish to use in to the "GameData\Squad\Flags\" folder of Kerbal Space Program
Default location for Windows is "C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\GameData\Squad\Flags"