A few years ago someone taught me an interesting way to sex ducklings by sound. This is not completely accurate, and it can take a little while to learn, but it is worth it. At ~$5 a duck, and straight run (not sexed) at that, you might as well try to get what you want, especially since I don't see prices going down anytime soon. Ducks used to be really popular--long story short, they were raised near water and fed fish; customers said "I can already eat fishy meat called fish and it's cheaper"!
Basically, all you do is pick up the duckling and listen to what kind of noise it makes and when it is made. If you hear an immediate high pitched noise chances are good it's female. If it makes no noise at all, it's a male. If it makes a raspy noise it's a male. If it's quiet for a little bit then makes a high pitched noise, it's possibly a male. I have been pretty successful with this technique so far.