+1 on Tractor supply. If you call first, don't ask for black sand, ask for black blasting media. I made the call asking for black sand , was told don't carry it. Then I found out Black Beauty or Black Diamond was blasting media.
Also, call all the masonry yards in your area.
Peninsula Building Supply is a big one, with yards in Livermore and Santa Clara.
There are others. Look for masonry, brick, rock, soil, bark, landscape and related terms.
Is there a Tractor Supply down there? The nearest one I know of is in Dixon or near Petaluma.