Louis, I've never seen precast slabs that big but they may be available somewhere. Generally for something that big people cast them in place. If you're wanting several of them I think doing it that way is the best option, they can be aligned and leveled easier and more precisely than precast ones and I don't think the cost would be too great. In CA it's pretty easy to find concrete guys who are willing and able to do a job like this.

anything that big is going to be cast in place.