To complete your collection of rare cards, you must have defeated CC Group’s King at Balamb Garden either in Disc 2 or 3. If so, meet up with Xu at the Entrance of the ship. As for the rest of the CC Group, they’re scattered all over the Ragnarok. Note that the Joker near the elevator sells you items, in addition to providing weapon remodeling services. To play King, put ‘him’ in your main party then talk to ‘him’ using [Square] while in the Air Room of the Ragnarok. Each CC member plays according to a certain region’s rules:

Jack: Aisle near Entrance (Balamb rules)
Club: Aisle near Air Room (Dollet rules)
Diamond: Hangar (Trabia rules)
Spade: Hangar near the save point (Esthar rules)
Heart: Entrance (FH rules)
King: Air Room (Lunar Base rules)
Joker: Aisle near Passenger Deck (Centra rules)

The CC Group holds virtually all the rare cards you’ve yet to collect, however they won’t be your only source of rare cards in Disc 4. The Card Queen can still be found on the Overworld near one of the southern tips of the Esthar continent, in the Abadan Plains. Run to that specific area to enter a field screen in order to see her.