Trade 10 Rusty Subligar to Heiji, who is situated at the top of the ramp, just to the left of the large staircase in the main room.
Despite the quest description text, he will accept multiple subligaria in each trade.
People seem to misinterpret the in game text. He does NOT say he will ONLY accept them one at a time. It's meant to say that he does not expect them all at the same time, which, since a trade window only lets you trade 8 at once, makes sense. Some quest require you to turn in all the items at the same time, they won't accept them item by item, that is what they mean by will accept them one at a time.