Take a look at http://www.vacationstogo.com
Also, you may visit the cruise lines directly. I recommend allowing yourself at least a week to book, before the date of the actual cruise. They need time to get your information and clearance to Imigration. I don't believe you can simply show up the day of a cruise and expect to book it. That used to be possible, but not anymore. If you plan to take a cruise this month, it's best to start looking around and booking as soon as possible.
It probably wouldn't hurt to get travel insurance Just.In.Case something came up at the last minute and you needed to cancel. Without it, cruise lines normally don't provide refunds if you needed to cancel, if it's so close to cruising time.
Have a wonderful trip!