If you are near a river or creek with clay banks you can use the clay from there. Or you can pick up some cheap kitty litter, read the contents should say bentonite clay with no deodorant. Mix it with some water to make a stiff batch mixed with a little sand. If you have a pottery supply near you, pick up some fire clay. Any of those will work.
Not knowing where you are located that's about the best I can do. Have you read this yet? READ THIS FIRST