Orbits and Travelocity are travel agencies. So is Cruise.com.
But always use a travel agent to book a cruise. He/her is a third party who will be more impartial than a cruise line. The cost is the same, although sometimes one agent is lower becasuse they're selling a specific cruise as a group. You won't know you're part of a group, it just gets you a better price.
Remember, the ship you pick is the first -- and most important decision. Then go scouting around for the best price on the ship of your choice.
But ships are going out full and people are booking far in advance now so steep discounting is a thing of the past.