I've never seen clam chowder on any menu in SF, though I suspect some of the places down in Fisherman's Wharf might have some. More likely than not, the clam chowder down there comes frozen from a big national food supplier like Sysco Systems and will not differ from the clam chowder offered at a diner in rural Montana. I'd love to know if there is anything worthwhile, though, as good clam chowder can be nice from time to time.