I'm not sure if links are allowed, but I like to use the cruise section of American Airlines. Go to their home page, then Plan Travel, then Cruises. Once you get the search narrowed down to a particular cruise, it shows you a list of available cabin categories and prices. If you pick one of those, it will show you a deck map and highlight available cabins in that category. The nice thing about the AA site, unlike many others, is that you don't have to create a dummy booking with passenger names, etc before being shown the list of available cabins.
One thing to note: It's my understanding that this will not necessarily show all available cabins in a particular category, and that there's an upper limit as to how many it will show. So, you see a representative sample, and everything shown is available, but there may or may not be more available. Among other things, this is done I think so that cruises that are far off in the future don't look completely empty when they are first opened for booking. It's also my understanding that this is the set of rooms that TAs will see through their booking systems. If you really want a particular room, or have general requirements that are not satisfied by what's available (such as needing connecting rooms), then your TA has to call Disney to see what is really available. Some don't want to take the time to do this.