Looking for toilet paper? Good luck. A surge of new coronavirus cases in the U.S. is sending people back to stores to stockpile again, leaving shelves bare and forcing retailers to put limits on purchases. Walmart said Tuesday it’s having trouble keeping up with demand for cleaning supplies in some stores. Supermarket chains Kroger and Publix are limiting how much toilet paper and paper towels shoppers can buy after demand spiked recently. And Amazon is sold out of most disinfectant wipes and paper towels.