I have found a couple of pizza stone options, after experiencing the
cracking you are talking about. I went to Home Depot and found some
12x12 clay tiles, I believe the model was Sausalito, and they were
unglazed. Home Depot will even give you a few tile cuts for free, so
I bought 3 tiles, about $6.00, and cut one tile in thirds. That gave
me a 12x16 surface area when I combined a full square tile with a
third size cut tile. Dry it in the oven at a low temperature to get
most of the moisture out and it will likely not crack. If it does
crack, you have a backup ready to go. I am guessing you paid at
least $20 for your stone. At the unglazed tile price, that's at
least 6 stones.
The other option I use (and it is great for the barbeque), is a split
fire brick. A regular brick is pretty thick, but a split brick is
about 3/4" thick. Find a contractors supply or rock supply store
that sells fire brick, specifically split fire brick. I bought six
for around $11. I also bought some sort of stainless grill grate for
vegetables at a Linens and Things store for about $15. I placed 3
bricks on the grate, and then laid down 1 1/2 bricks perpendicular to
the three and I have about a 12x12 surface that lifts all in one
piece thanks to the grate it is one. While I have not tried that set
up in the oven, it should work with maybe a little extra heat up time
due to the thickness of the brick ( about 1")
Good luck!