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.
Cut Um! Real ducks don't have a competition sequence they follow! Hint, hint!
My first goose came down to a 7/8 ounce load of 2s. I still remember it, it was the farthest I have ever killed a goose to date. My dad couldn't believe It. Shoot them in the lips though, cuz it isn't a total penetration load. If I were you I'd look into heavy shot, yes it is expensive but with the 20 it may be the difference
Keep In mind too, my dad has hunted wood ducks for years with a .410 and a whopping payload of 3/8 of an ounce of 4 shot, and I have seen him shoot geese with his little .410 and fold them dead. He just has to be super selective and wait for them to get close enough to see their eyelashes which he then aims for. Moral of the story, a smaller caliber isn't necessarily a hinderance. /end rambling.