We buy it from either butchers, markets and Waitrose even do it (probably not at great but will do if we can’t get the others) We soak in ice water overnight, oven cook it, then whip the marrow into butter. I then use the bones for making broth with other bones. Whipped bone marrow should last up to a week in the fridge and 6 months in the freezer so you could portion it out into daily size bits and take them out each day if you’re worried about it spoiling.
I don’t think I’ve ever “eaten it” but PHO is my country’s most popular dish and that’s a meal of mainly bone broth, so still full of collagen being cooked up w bones so even though it’s not straight marrow it’s really good for you.
We’ve always got it from the farmer’s market and cooked it along with our steaks. It’s not a common thing we eat, but if it’s available we’ll pick some up!