Once you have a pretty good idea of how many, what ship, and when. Call Carnival's 800 number and price it. Then call again and talk to group booking and price the same thing. Then talk to a Carnival PVP [personal vacation planner] and price again.
Then click on ALL the various TA's with adds on the board. Some have online booking engines and some don't, but for multiple cabins you will have to call them anyway. Call them all and shop, shop, shop. Then talk to your local TA's and/or visit websites of other TA's you have heard about. If you want the best deal, it takes some effort.