There would be Drogon, Rhaegal, and Viseron as Danaerys original three, Drogon is still around and kicking, being close to his maximum age, as they don't live very far beyond 200. So, the original trifecta of Dragons are definitely the greybeards, and may go at any time, if not already having passed. That means the newer generations of dragons are the ones to be looking for. My two are both three years old, leaving perhaps a handful of others that are out and about. For those, their whereabouts are up to the head honcho, if they are currently controlled by a rider, have gone rogue, or just flew off into the sunset, so to speak.
That leaves the newest batch of eggs, being roughly 3 years old, left, if not any older ones, whose whereabouts are not fully known. Based on traditional European Dragons, they lay 3 to 5 eggs roughly every 10 years. Your call on this one GM and Co-GM.