I've heard back from Robin that she doesn't know of any stories with a female Flagg so my gut feeling is that there isn't one and the person at the tour company is mistaken. With as much research as Robin has done for the Dark Tower Concordance, I can't imagine she would have missed it.
Sounds like a mistake to me. The fact that a guide, who should know his facts has forgotten which story it is, says enough. Maybe it's some kind of wishful thinking - that they would like to see Flagg as a woman - especially since King has done quite a number of books from the perspective of women.