I think you need to find a good travel agent. But I just googled " repositioning cruises" and you get different websites that have different search engines that you may want to play with.
Normally these are at the end of a season when ships move from one zone to another, they include many sea days and are less expensive
Ships that are in the Caribbean for the winter will move either to
Alaska
Europe
East coast for the summer.
.
Example. Holland America has one ship that goes from Florida up to Montreal early May.
That ship then does Montreal to Boston for the summer. ( This year was the Zaandam ).
Think of were the ships are which month, and when they would move back.
.
If you are set on carnival. Look 1 ship at a time for a one year period. You will soon see the pattern.
Which I could give you more info, but the rules don't allow it