NAPA Auto parts sells Floor Dry which is what you're looking for. Other auto parts stores might have something similar. Walmart has Oil*Dri which is a clay product, I don't know how durable it is but supposedly it doesn't break down.

Thanks haname:

I looked up the manufacturer of Floor Dry. They are the same that make the Axis brand soil conditioner I found on Amazon so Floor Dry and Axis might be the exact same thing, although I'd hate for there to be additives in the Floor Dry since it will be used for vegetables.

I'm going to email the manufacturer and ask.

Thanks again.