I agree with Villager - VTG is a good research site, but after you find the single sailing you're interested in, then contact your agent.
As for the group you encountered that had a better rate than you, that's not unusual. Sometimes agents will book a group many months or even years in advance and can get a group rate that is cheaper than the going rate. And if that particular sailing is popular, the rates could go up making that group rate even more attractive. For example, our agency pre-books over 40,000 cabins into group space every year. If you pick a cabin on one of those sailings, often you can not only get a better rate, but also some perks such as onboard credit, pre-paid gratuities, bottle of wine, dinner in a specialty restaurant, etc.