It is no more expensive to book through an agent than to do it alone. The agents will quote the same rates as the cruiselines are offering at the time. The only difference is that SOMETIMES the agent will provide a small bonus, like a bottle of wine in your cabin or a bit of on-board credit.
Check out whatever agents may be close to you; if they don't have lots of cruise line brochures on display, they may not have as much cruise experience. Or try one of the on-line agents, like Vacations to Go that has been frequently mentioned on these posts, though they won't offer you any bennies unless you specifically ask about them.