There is a small high quality feed store in Olympia called Eastside Urban Farm and Garden that has black runner ducks right now. I did not check and see if they were sexed, and I don't know if that would work for you anyway, or if you would want to even drive that far. But we've had really good luck with their chicks all being properly sexed and much higher quality than TSC and the like.
We want to get an Americauna rooster, but not right now. We have one rooster already, and 16 hens/pullets. We're strongly considering placing an order with Dunlap hatchery in the spring. Last spring we ordered hatching eggs from them, and even with our crappy incubator the regular sized eggs had a 50% hatch rate. The bantam eggs did not do so well. But their little bantam Americauna eggs were just a gorgeous shade of turquoise. Theoretically if we ordered chicks they should be wonderful. They also have bantam blue laced red wyandottes. And Indian runner ducks. No clue on sexing ducklings, but they were really nice when we ordered from them before over the phone.
Sorry, I haven't been able to figure out how to insert a quote yet. This was in response to yesterday's inquiry about finding an Indian runner duck in Western Washington or a good hatchery to order from.