Next time, take a bigger suitcase
Just kidding, I really have no idea where you could get a different seat. I have a funny feeling you'd end up replacing the whole toilet in order to make the seat work. Next time you do go on a cruise though, see if you can find the manufacturer name on the toilet and write it down.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR