In the Dunk and Egg novella The Mystery Knight, Dragon Eggs are featured prominently: one of them is the prize of the Whitewalls Tourney hosted by Lord Butterwell, and Egg himself states that he (and his brothers too) also possesses one of them, that was put in his cradle when he was a child.
At the end of the story
we learn that the Targaryens, via Bloodraven, acquire also Lord Butterwell's Egg.
If all the children of a cadet Prince of the Royal Family all possess a Dragon Egg, I think that we could safely assume that they are (relatively, of course) common, or at least not so rare.
We know that seven of them
were probably destroyed during the Tragedy at Summerhall
but what happened to the other ones?