I think that Carnival has an abbreviated wine list in the Gifts section of their website. You could look through that and try to find the similar wines locally.
George Debouef bottles some excellent, moderately priced wines.
By the way - if your friends bring a gallon jug wine on board, it is not likely that anyone will think they bought it on board. The wines sold on board are in smaller (750ml) bottles, for the most part.
If you really want to break their chops, while they are going through the pretense of offering up this wine as having been obtained on board, make a point of telling them that you think it has turned and that it should be sent back. See how they react to that