Thanks, both. I managed to swipe the one from Nettie’s table without her noticing. The one in the grass outside is also owned by the druids and looked much harder to get away with stealing!
Simply throw the sickle lying on the ground away from the area of the magic ritual. Then one of the druids comes and collects it. You only get a -5 reputation penalty, but nobody cares. After that you can legally buy the sickle from this NPC (I forget his name).