On our past two cruises the tickets for the princess gathering and Elsa & Anna gathering were at the Shore Excursion desk once and Guest Services once (which is where I'd check). If you're going before Christmas tickets for the gingerbread house building are at Guest Services. There haven't been tickets for character gatherings - all of the characters are available separately several times during the cruise. In any case, if you have to get tickets, do it as soon as you board - they go fast!