Let's not forget the largest collection of flags collected on the web : Flags of the World, maintained since lots of years (initially via Usenet before the Internet we know today). All other references are found there, including the International Association of Vexillologal Association (IAVA), that should be involved in the project of building and maintaining a registry of flag codes.
The FOTW seb site has always had several domains, some disappearing, but mirrored together. This one is the most stable : http://www.crwflags.com/fotw/flags/index.html
