Moler clay is the main part of cat litter, it's the physical grains that you see. Cat litter usually has various additives and perfumes in it too, so you can't use any old cat litter (the Tesco one that Michael mentioned doesn't have additives in it).
If you want it in bulk, you can buy sacks of it from bonsai suppliers (I've got two huge sacks in the corner of the dining room, more than I'll ever need!).