I know this is an old thread, but I have to add that Prather Ranch produces 100% grass fed beef (and they demand the same of their partner producers). Prather has a shop in the Ferry Building in SF that is open 7 days a week, and I have spent some time chatting with the guys behind the counter. They're serious about animal husbandry practices, and serious about good beef and pork. Occasionally they offer pasture-raised chicken and eggs also. Yum! And I don't feel guilty about eating it.
Niman is a great local company, but they do finish their animals on corn. However, they have a great pork program-- you can read about it on their website.