I have used fire bricks for about 5 years now. They can be purchased from a building supply center for a couple of dollars each. They come in two thicknesses...I use the ones that are 3/4" thick and nominally 4" x 8". They stand up to the barbeque....I have backed dozens of loafs of bread with the barbeque on full speed. They are light yellow in color and are the same bricks that are used to line fireplaces.
The only down side is that, even if you place them tight together, cornmeal tends to dribble through into the rest of the oven....I fix that by placing a some aluminum foil below them...or sometimes double up on the bricks with the joints staggered.
I plan to make a metal frame and maybe morter the bricks together... I figure that will be pretty close to an actual hearth