Bi-Mart, Cabelas, Skagit Guns, Sportsman's and if you order on line I would go with Rodger's. They will deliver to your door free of charge, no sales tax if you pick certain cases of shells. Other good 20 gauge options as well, but I try to keep it consistent for the kids, so their leads don't change.
Real ducks don't have a competition sequence they follow! Hint, hint!