Good marrow bones are hard to come by. I asked the local butcher and he told me that the dog bone companies buy them up from the slaughterhouses & stew them and sell each one for $ 7-15 apiece.
I was able to finally locate a source. The marrow is the best part, of course.
What is also difficult to obtain are organ meats, particularly kidneys. Gots to go out into the country for that. Really good stock takes at least two and preferably three days to make.