Some of these flags have been obtained using sodipodi to convert the sodipodi flags collection from svg to png and then modified a bit (sorry) to be of similar sizes. Sodipodi flags collection can be found at https://web.archive.org/web/20080116221835/sodipodi.com/index.php3">http://www.sodipodi.com/index.php3?section=clipart/flags Those flags are licensed under the Creative Commons Public Domain license.
The rest have been obtained from www.wikipedia.org