It was not a museum but rather Jeff Bridgman's web site. He had one for sale some years ago. You can find it by just Googling "Seymour Blair campaign flag images." it is based on the Stars and Stripes however.
Since the Democrats were segregationists as part of their platform (you will also see when the campaign flag comes up a handbill that mentions the US being a white man's country and let white men rule it), I can see where some campaign flags might have been made using CS First National flags made for the purpose.