I bought some red iron oxide off eBay, I haven't tried it yet but I am pretty sure is basically what your looking for. The red clay is used because it is rich in iron. You may want to seek some more opinions first of coarse. 10lb for about $22.
I went into Michaels in Livonia today and I found Natural Red Clay, it is not on their website nor could I find it on the manufactures website either so you may have a tuff time getting it still. Also the store clerk on the phone was not very useful.
You could try and call ahead and ask them to specifically look for a 10lb box of Natural Red Clay by Amaco, but like I said I cant find it listed online at all just on the shelf so its a gamble. They may be able to order it for you if its no there.