I have recently been wondering about local sources of (free) clay. So I googled it.
One recommendation was to go out to construction or street work sites to find exposed clay. Another said that clay can be found two feet below the surface in most places in the world.
My ultimate goal is an earthen oven, so I was glad to find these tips:
We have found that clay of some sort can be found almost anywhere. However, finding clay that will meet stove construction requirements is demanding. Ideally, it should be easy to dig and to separate from other unwanted materials, e.g., sand, gravel, roots, leaves, and the like. An ideal clay would be quite plastic when moist, would dry and fire with a minimum shrinkage, would fire successfully to high temperatures and should retain its strength and integrity when subjected to repeated severe thermal shock. That’s a lot to ask and even in the finest commercially available clays, the combination of these traits is a compromise.
We have found that the most promising places to find clay are in exposed ditches, the banks and bottoms of
ponds, road cuts, excavated foundations, exposed roots of blown down trees, and the subsoil of marshy, poorly drained areas.
A spade, pick ax, and a small soil auger are useful in securing samples that can be tested. While it is satisfying to find your own clay, you can save a lot of time and effort by making friends with local potters. They can tell you where to dig or to buy the clay they use. Ceramic supply outlets are also very helpful in sharing information on the clays they sell.
I find that clay deposits can vary wildly in the space of mere feet. Being familiar with local geology is also helpful, but that usually requires staying in one place long enough to know the land.