Regus will do in a pinch; they have one on Folsom Street near Caltrain. Don't expect it to be cheap, but they're usually not full. They (and others like them)

Regus will do in a pinch; they have one on Folsom Street near Caltrain. Don't expect it to be cheap, but they're usually not full. They (and others like them) have spaces areawide; you may find some discounts closer to Burlingame or San Mateo, as there's slightly less demand outside the twin orbits of San Francisco and Palo Alto. If you're bootstrapping, you should defend why you're in San Francisco to begin with. Your runway is far shorter in SF; unless what you're working on will only work if you're there in person, or you're square dancing with funders each day, focusing on working almost anywhere else in the world would be smarter.