Not sure of a brand either, it is often a generic type. Just look at the ingredients. It should say: "Ingredient: Clay." Nothing else. No perfumes, chemicals, odor crystals, or anything else. If your local Mart doesn't have any, look around in the auto area for "oil-absorbent", and look at the ingredients. You may be able to find pure clay there.
There is quite a debate about the need to add pure clay, and what type(s) might be needed, but I assume that you've looked at your options and are comfortable with your choice. From every article and post that I've found, it sounds like at least one type of "pure clay cat litter" is available at any "Mart" (Wal-, K, Fred's), and other similar stores (Target), even some grocery stores. Also from what I'm reading, it is generally hidden low on the shelves (maybe behind a display of rhinestone collars), is generic-looking, and is cheaper than any other option.
Hope this helps! I'll be interested to learn whether it is as easy to find as the posters and authors think!! Lots of luck!