It can be obtained as drop loot from monsters or by pickpocketing a H.A.M. Member (requires level 15 Thieving). Giving Tindel Marchant a rusty sword is also a task for the Easy Ardougne Diary.

How do you get the rusted sword?

Rusted Sword is the held item for the legendary Pokemon Zacian. You will receive the item Rusted Sword once you capture Zacian in the post game.

What sword gets rusted one piece?

It was only destroyed due to rusting power of Shu's Sabi Sabi no Mi. When used by Zoro, Yubashiri was one of the three swords for his Santoryu style and it was typically used along with Sandai Kitetsu in his Nitoryu techniques.