While rare at standard grocery stores, you can find it through specialized retailers: Spinalis Dorsi, the Ribeye cap - Rare Steaks - Jess Pryles
The ribeye cap (or spinalis dorsi ) is widely reviewed as the most flavorful and tender part of the cow, often described as having the rich marbling of a ribeye with the tenderness of a filet mignon. Because it must be trimmed from a whole prime rib roast, it is rare and often pricey, but reviewers generally consider it a "must-buy" when found. Where to Buy Rib Cap