I have looked a few times in Walmarts and not found duck breast, which is something I eat in Australia once every 2-4 weeks. And also noticed particular cuts of beef that I'd expect are missing. I have been to the US 10+ times, usually 1-9 week roadtrips, and can't remember ever noticing an independent butcher (though sometimes there is an equivalent within a non-Walmart grocery store).
Duck just isn't commonly eaten in the US. You probably won't find it at butchers either.
Independent butchers aren't all that uncommon. Though, for example, my town has 3-4 grocers that sell meat without having a counter (but I expect you could talk to the butcher at a couple of them), a grocer that does have a counter, and then a couple of independent butchers.
Duck probably isn't eaten here all that much, but at my local supermarket there'd be breasts, Peking Duck flavoured breasts, confit legs, and another Peking Duck kit.
In that not-overly-large shopping complex, there are two supermarkets (one of which has loads of European smallgoods, like 40+ types of cheese) and also: independent greengrocer, butcher, fishmonger, bulk grains store, bakery chain, etc. Used to be a poultry-specialist butcher too. Most shopping centres I can think of here will have an independent butcher, plus more separately outside those. There'd be dozens of butchers in a city of about 1 million.
Your best bet for duck related products in the US is probably either an upscale grocery store or an Asian grocer like HMart. They tend to carry a lot of meat and offal products that most groceries in the US don't normally carry.
