I don't know why the NPS would say that the reason they don't have any Confederate battle flags for sell at the Vicksburg museum is because there were none at the siege. The NPS began removing the battle flags from all of their museum book stores in June 2015 because of political pressure. Here is one story concerning Gettysburg. https://www.newsweek.com/national-park-service-removes-confederate-flag-merchandise-gift-shops-347009