John:
While I think it would take more than a specialized list to start a stock photo business specializing in flags, it sounds like it has really interesting possibilities.
I doubt you'd be able to make a go of it on editorial sales alone, but as the flag specialist, you could also produce calendars, books, and other souvenirs as well as the odd (but very lucrative) commercial sale.
Let's remember that flags are very powerful symbols. We Americans venerate the stars and stripes, Canadians their maple leaf, and when I say "union jack," do I even have to mention what country it represents?
Every history book uses flag photos and it may well be that a better source for them could be welcomed. You can't find out without trying.
Brian Yarvin
Author, Photographer, Educator
[
www.brianyarvin.com]