TL;DR Costco sells as good as or better sourdough bread at an overall lower total cost than any household who doesn’t bake several loaves a week.
Regardless of where a sourdough starter is shipped from, it will localize to the place it lives.
San Francisco Sourdough bread will not be fully duplicated outside of the San Francisco area because the water is different, the air is different, the beneficial bacteria is different.
Get a starter, it’s not quite the demand of a cat but plan on feeding it at least twice a week. Buy the 50 lb bags of flour so you won’t cringe quite as much at the cost of the flour you are tossing twice a week when you’re not actively baking.
Like the cat, if you’re going to be gone from home for several days, who do you trust to feed your starter while you’re gone?