I thought that initially too, but then thought maybe I was wrong and my eyes were playing tricks on me. Regardless you can get Miao Dao that are single-edged, but like I said before, most of them are slightly curved just like a Katana. What the OP is looking for appears to be very specific to the art he is doing, meaning he will have to go to a korean weapons specialist or a custom-made sword.
All this has made me wonder though, why aren't there more single-edged 2-handed swords around? The tradition throughout history has either been a single-edged 1-hander or a double-edged 2-hander. Can anyone give a reason for this trend?