Cheapest non-sulfur (NSP) clay?

So, dumb question here. I see that everybody uses clay for two part molds, and some folks are making really big molds requiring a whole lot of clay. They always list the type of resin and silicone they use, but never the clay.

I'm looking for the cheapest non-sulfur-based clay out there. I'm going to be making some really big two part molds, and I'll need tons of clay to do it. Preferably fairly soft clay. It doesn't need to be that hard sculpting clay, cause it will be easy to use if it were softer; I wont be sculpting with it. What are your guy's/gals' experiences with shopping for clay for this purpose?

Klean Klay is the cheapest I've come across. You can buy directly from them by the case and get the sticks unwrapped. Not the best to sculpt with, but good enough for your purposes.

Klean Klay went out of business some time back. Becoming impossible to find the stuff. Too bad as it was great for these purposes.
Closest replacement I have found is:
Sculpture Supply Canada - Product Detail