outdoor oven

Help!

I've started building my outdoor bread oven from Denzer's "Build your own earth oven" book (the base has come along nicely!) but I didn't realize I was going to have such a hard time buying clay or a clay mix for the oven mix. I was hoping that digging on my property would turn up a decent mix, but it was all sand.  I've called around landscape and brick suppliers and can't find any powered/dry clay or 'builders mix' that is specified in the book.