You must Quest for it.
The Ancient Katana is an old Monthly Collectible released in July 2006. Due to its age, high demand, and popularity from its release. It is worth thousands in Gold.
If you have skills in art, you can create a Mini Shop and sell your work for Gold. There are also Dice roll contest run by Users that give you the chance to win a lot of Gold or the very item you've been Questing for. There is also Booty Grab as a way to earn Gold fairly quickly.