You need 500000 gold to buy this. It is only available to buy once you clear the game. To clear you need to do a full run through the normal castle and the inverted castle and then the credits will appear and you will be back on the menu. You can then load your Clear save file and start grinding for money.
First thing I would do is go to the long library on the right side (normal map) as seen here with the red arrow
and sell all the items you can to the librarian. Also buy some Library cards so you can teleport back here.
Now head to the Colosseum and find the room shown with the arrow on my map. It is near the middle, slightly to the left.
You want to go in this room with the Jewel sword and 2 Ring of Arcana's equipped. The Jewel sword is required as it has the chance of turning enemies to jewels. The Rings increase the chance of getting rare Jewels.
The Zircon is the most common drop. You should keep a check on how many you have by going into the menu, selecting Equip, and checking one of the 2 Other slots to find your Zircon count. If it has hit 99, Equip a Library Card within your Left or Right hand and use it. This will teleport to the library where you can go back to the librarian as shown on my first map and sell everything. Repeat this process until you have 500000 gold and can buy the Duplicator.
The following video is the most thorough I could find. It explains how to get the Jewel sword if needed, also an item to increase luck if you want. I didn't bother with that as I figured by the time I got it, I could have grinded out the gold. It also includes other methods you could use: