It’s pretty hard to find bone marrow that’s already been removed from the bone, but if you want to poach it, sear it, or use it in a confit, you need to get it on its own.
Luckily, Niman Ranch sells it online. It comes frozen, in 2-lb. bags.
Scroll down to code 95-1 in the Beef Offal section of their product list.
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Bone Marrow