I think these olives are called 'castaveltrano' but I've also heard something like 'castalaverde'. They are round, green and buttery, smaller, richer and more flavorful than the cerignolas you can get now, much less astringent than most others. I've seen them in only two places in the city: the big market at Arthur Ave (the guy across from the salumeria), and at the food shop at ABC Carpet & Home, where they were sitting in an unlabeled bowl next to the other olives. The guy at the counter there thought they were regular Greek olives. I tried them and they weren't. Yay!
Nobody else seems to carry these -- I've looked at Fairway, Gourmet Garage, 9th Ave. Cheese, and Zabars. But if you see any, grab 'em, and let us know where you found them.