If you mean private excursions, then google up the port of call and research. Thats how I did it. Most ports and vendors have sites and links set up for you to look at. As an example.. I put in Calica port excursions or Calica excursions and hit search..
You can also research with any search engine you have. I looked at the list Carnival had available then searched for them outside of Carnival to see what the price differences where. Sometimes it can be significant.
Life is too short to let the ship of your dreams sail without you.
Carnival Destiny Feb. 2006
Carnival Fascination Feb. 2007
Royal Caribbean Grandeur of the Seas Feb. 2008 The DTW & MsBJ tour
Carnival Valor MsBJ and Dina Feb. 2009
Carnival Glory MsBJ and Dina Feb. 2010
Carnival Dream MsBJ and Dina Feb. 2011