I don't know if "exotic" is the right word, but they are scarce; there aren't many good marrow bones compared to the amount of meat on a cow. I have good relationships with a couple Chicago butchers (I was an investor) and I'd have to ask far ahead for the marrow bones. Since there's so little of it to sell, they're more likely to make a product that stretches them, like marrow butter or bone marrow burgers, than to simply sell the bones.