I can't confirm any of these spots, as I pump my own, but I would try down near the docks, where they rent crab rings, etc. The hardware store used to carry bait, but I don't know if they still do. Also, there is a place just east right as you go over the slough that advertises sand shrimp (and crab, bait, etc.) Upriver a bit, there's Oakland's that probably carries them as well. Good luck!