[Cob] Curious about the clay

[Robert J Churchill]

The word "clay" is used repeatedly but really where exactly is all this clay coming from?  I have no idea where I'm supposed to get all this clay.  For me, it would be easier just to buy clay from a ceramics shop but I don't even know if that would work.  I live in an urban area and it's not like we can just go out and dig up clay at a local quarry.