I'm planning to try to toilet train our two cats..and I want to start using a litter that is septic safe. I don't intend to dump any down the toilet, but with the insert (the litter kwitter system) I know some litter will accidently get in the toilet. So what would be the best for this? I want something I can buy at Walmart, Petco, or Petsmart, I don't want to have to buy it online. Thanks!!