I agree with Sonny that if you're happy with the service, there's no reason to switch agents.
But to answer your question as to how to find a cruise travel agent, I highly recommend seeking someone with certification from CLIA (Cruise Line International Association). Their Web site (http://cruising.org/index2.cfm
) has a search function on the lower right to help you find agents in the US and Canada who have passed their intensive certification requirements. Enter your postal code (or part of it, for more results).