With the changes in cruise line pricing/advertising policies in the last two years you can unfortunately no longer find the actual best price a travel agency can offer by surfing their web sites.
Though they can't advertise prices cheaper than what the cruise line is advertising them for, THEY CAN sell them for less, or offer other incentives like shipboard credits, etc. if you contact them directly via e-mail or phone call.
It takes a bit more work to find out what the best prices actually are, but the savings can be significant enough to make it worth the effort.
Save yourself lots of time and contact a good travel agent. A good agent will help find the right cruise for you, what cabins are good to book, make payments for you and handle any changes you decide to make. Agents are not allowed to add fees to cruises. Ask for a copy of your cruise line reservation. (not their agency receipt)
there are many web sites for booking cruises at great prices. I recommend CPI king on the tab above that says "cruise shopping" and check out every link there. You will be quite surprised at the bargains there