My duckweed came from plants that I bought from another ponder. It is hard to seperate them from the roots as they cling when removed. I remember asking for some and he said, "don't worry, you will have plenty soon enough".
They add a nice touch to any pond and are easily controlled by netting, I don't think you will ever get rid of them unless winter kills them. That said, they do offer shade to the fish and an occasional healthy snack.