For one of my research projects I needed the flags of all countries. These national flags are often available as icons or bitmap graphics, but it was difficult to find them in a vector format, such as SVG. Here are some sources for bitmap flags:
Wikimedia has a full overview of all the flags with links to the flags in various formats. But I needed them all together on my computer for processing, so I harvested them all an put them into one zip file. All the flags are under Creative Commons License, so feel free to use them. You can download the All National Flags In Vector Format (SVG).