Thats Kaolinite. But make sure it has a red/orange hue to it, because some Kaolinites are iron free with a white/grey look.The clay is kaolinate i.e. potters clay. As long as it is just the ingredient is just the clay you can pick it up at any craft/art supply store.
I store them in ziplock bags, and keep them on my shelf with dry fertilizers and plants misc.Awesome...and then as for storing, do you just put all the balls in a plastic container under your tank?
might want to try ac moore for your red clay (cheaper than joanns fabrics) or talk to a nice pottery teacher at a local elementary school and she/he may toss u some for free...i mean they buy it by the hundred pounds..