I got some firebricks at my local landscape stoneyard. Love 'em.
And Tim in PA, when I was there I was asking for ideas for plausible pizza stones... I asked what site-built pizza ovens used for floors, and the guy told me that his company sometimes carries high-temp concrete, but it always sells out really quick. Do you have any thoughts about this? If we could source some high-temp concrete, we could build some forms for pizza stones...