I would get on the phone, not the internet.
Try calling Morgans in Antioch and Brickyard in Concord. Ask if they have it, but if they do not, ask if they know anyone who does.
There are also several rock yards out in Brentwood, one on the far side at Byron Hwy, the other north of town on the old Hwy 4.
Try other construction stores, too. Maybe lumber yards?? How about White Cap? Fairfield and Concord are the closest that I know of to Antioch.
There are a Home Depot and a Lowes in Antioch, way out on the new part of Lone Tree.
Lots of phone calls!
Tractor Supply in Dixon is a bit of a drive.