There's a grocery store on 24th Street in SF, near Alabama Street, that sells it. It's on the south side of the street -- the butcher is in the back of the store on the right. I can't remember the name of it.
I bet most of the grocery stores in that area sell it.
I bought a pack of the Chorizo Verde ($12) last weekend at the Healdsburg farmers Market and grilled them over mesquite for some friends. These are made with heritage pork, spinach, pumpkin seeds, cilantro and serrano chiles, and have a lovely bright green color when raw.