I totally lucked out at the Berkeley Bowl today. I went for my usual grocery shop and had it in mind that I would like some beef stock. I asked one of the ladies at the meat counter if they had any organic beef bones in the back. She seemed dubious, but It turned out they had a femur! I snapped that up quick and had them cut it up into reasonable sized pieces. Now I am slowly filtering many many cups of water through my Brita for soup.
The simmering of the beef bones can go between 12-72 hours, depending on who your natural gas supplier is. Mine is PG&E so I'll probably simmer the bones around 20 hours.