You wont find much, if any at Walmart, but any shop that stocks 28g usually has 16g as well. With 3in magnum 20g loads out there the 16g just doesn't have the appeal it used to.
My grandfather had a Remington model 11 16g and it kicked like a mule. Unfortunately a POS cousin of mine stole it and pawned it for dope along with a few of my grandfather's nice old guns. I did manage to get his Browning Belgium made .22 takedown before he got his hands on it.