Agents don't need to call every hour - all we have to do is hit the reset button and it redisplays the information - quick, simple, and easy. Plus, if you have an agent, between the two of you, you can cover alot more ground than doing it by yourself. And if you book directly with the cruise line and have problems, who do you think the cruise line represents?
But if you can tell us what ports, dates, and lengths of cruises you're most interested in and perhaps people here can help lead you in the right direction because a quick search shows Carnival does have cabins available - may just not be on the ship and sail date you want. Might also want to consider other cruise lines as there is also space available on several others.