While some sites do offer discounts for some cruise lines, Disney does not allow it at all.
Quite frankly, if you have a good Cruise Specialist, contact them, let them know what you're looking for, and let them do all the research for you to find the best deals.
As I posted in another thread;
I'll be honest and say I'm not a big fan of those large online booking sites for the reasons CLIA (the Cruise Lines International Association) once stated. They said their investigation found that when booking with online sites and the cruise lines, you find that: 1) the person you're talking to works in a call center; 2) is not a travel agent - they're just an order taker; 3) has worked for the company for less than 6 months; and 4) has never even been on a ship or been to any of the ports, so their information is basically non-existent. Plus, booking with an online site with someone who is not an agent has inherent stress.
I suggest call an agent, preferably a Cruise Specialist, and discuss what you're looking for with them. They charge nothing for their services and their expertise can be invaluable. Plus, they'll have a great deal of knowledge and personal experience they can share with you that will enhance your vacation and make it more hassle-free.
But be very careful when shopping deals. Remember, price is what you pay - value is what you get. Sometimes best price can end up costing you alot in the end, especially if you have any problems and need assistance.