Local stores will normally order straight run, which means they get males and females and you can't tell them apart unless you vent sex them.
If you want a drake you should get at least 3 females. Sometimes you can have a mating pair but it is more likely that the female would get stressed from being over breed. If you just want the ducks as pets, and you only want two, I would recommend getting only females.
Metzer Farms has Blue Swedish. You can order a minimum of two ducks and they will sex them. You will pay more for the sexing and for a small order. I went with Metzer because I only wanted 4 girls and I wanted a specific breed that I couldn't find locally. I was very happy with the order.