In the past I have ordered a gallon of Clay slip from an Art store, such as: www.dickblick.com
They did not carry powdered clay, but the clay slip as described below worked well!
I live on a Barrier island in Southern NJ and digging down would only get me salt water or beach sand!
Here is the formula, which has been attributed to the famous slab builder, Joe Day:
Joe Day Recipe: The recipe I use for muck is powdered clay and sphagnum moss with the moss being 80% milled sphagnum and 20% long fibered sphagnum. Dampen your moss and mix in the powdered clay until it forms a bread dough consistency.