Sites such as Travelocity or CheapTickets.com will show you some of the available staterooms. I think they will show you the 8 "best available" for a given category on a given deck. I think they base the best location on proximity to the center of the ship. I don't know of a web address where you can find all available rooms for a given cruise.
If you have a specific room or location in mind, your travel agent or DCL can tell you if it is available and reserve it for you.
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