Philippe prefers to use a low-fire (fires to cone 06), white earthenware (water-based) clay without grog. Clays are manufactured locally so one purchased in California will be different from one in New York. Ceramic supply shops are the best source for this type of clay. A few that Philippe has personally used and finds closest to the texture he prefers are:
A search on the internet for any of these clay types should provide various distributors around the US.