Firebricks work great and are low cost. You can usually find them at big box home improvement stores in winter or at stove and fireplaces year round. There are lots of sizes but the case I have are about 9x5x1 which is a pretty common size. I bought a box of 12 at a Home Depot spring clearance for $11.
They are also great for adding thermal mass to other types of cookers, for a cheap indirect method on ceramic cookers, or for smash cooking that perfect panini.