I agree, always a travel agent. You also have to make sure the prices quoted are with all the taxes, etc. A lot of the time, they will advertize a price that does not include that while an agency may have that figured in, so they look higher, but may even be lower.
Many cruise travel agents also buy up group space and can give you a better deal/price on any of those cruises, always ask about those. They also usually add something, like an onboard credit.