Grass fed beef in San Luis Obispo
Villa Creek in Paso Robles has featured it I know. I have also had grass-fed Bison there as well.
I agree with the Old Creek Ranch suggestion. We buy their stuff at the Templeton market all the time. I am sure they sell locally to restaurants also.
Old Creek Ranch in Cayucos sells such beef at the larger Farmer's Markets. Not at home, so I don't have the info. Google the topic and call to see where they sell locally? It's not going to be cheap.
Big Sky Cafe in SLO trumpets the use of local producers, but don't
know if they have grass-fed beef.