I think also that the petstores stock what they can sell. They can't let a bag of dogfood sit on the shelf for too long, so they are hoping the dogfood they put on the shelf will be purchased in a short period of time.
One thing I wanted to say, is that small specialty pet stores will order foods for you. They might not carry all in the store. I'm in Illinois and there are two stores close to me that will order just about any food for you. Both ship I believe anywhere in the US. None carry the brands that are carried in the larger chain stores.
I see you are in San Francisco so I would think there are some stores that would do that also