You can try hotels.com or priceline. You can get a lot of hotel deals online. Once you get a price you might want to compare it to the Hotel website, though. Many hotels say the lowest prices are on their own sites now.
Lots of the hotels have "park and cruise" specials so that you can leave your car at the hotel, be shuttled to the pier (often for free) and shuttled back to the hotel when you return.
CarnivalSensation February 2015
Carnival Dream November 2015
Carnival Fascination April 2014
Carnival Elation March 2011
Carnival Imagination Sept 2007
Carnival Sensation Dec 2006
RCI Sovereign of the Seas Sept 2006
Carnival Miracle Sept 2005
Carnival Glory Sept 2004
Carnival Fantasy Jan 2004