Can you buy a extra dining card for buffets etc

Those can only be bought with a ticket to the parks. Im assuming you're asking because you have a young child that doesn't require a ticket. You should be able to share with them easily. There's plenty of food in the dining plans.

They don't have dining cards. You can get a dining plan which is based on number of night stay.

Dining plans are tied to NAMED guests staying in the room. That said, the meals are pretty generous in size and sharing is definitely not out of the question.

Thanks