The ones I buy most often are 1 lb. bags from C & F brand – City of Industry, CA. They are in with the dried beans, lentils, etc. in our regular grocery stores. After soaking, they get big and rounded. The only way I can describe the flavor is very creamy and ‘fatty’ (in a good way), like being cooked with a big piece of salt pork, even if you just cook them in water.
They are used in PA Dutch cooking for bean soups but they use they big dried lima beans for baked beans, not marrows.
Now you’ve got me wondering if this is another one of those regional foods or if other places in the U.S. use them. [?]