I used
Last Minute Cruises for this trip, but you can find deals at several on-line booking sites like
Cruise Pro's
One thing I've learned from searching, more like wading, through these sites listing is the more flexible you are and the sooner to the departure date the better the deals. If you're dates are set and you can't find a cruise you want on one site try another. Google "last minute cruises," you'll get a bunch of site to wade through.