On the weekends, I often see very well dress men and women carrying around sandwich boards advertising open homes. I often see one or two come out of the back of a car, so I just assumed that each agent carried around his own sandwich board. What if you had multiple properties listed? That would be a lot of sandwich boards.
At least in the Castro, I found out where one company parks their sandwich boards – in a garage, of course? I was walking by the terminal stop of the F historic street car, saw something in the garage that caught my eye, and there they were. There were a ton of them inside the garage.