How do get the Stardust Sword on Dragon Quest IX?

Answer 1

Stardust Swords are found in high level grottoes. Sometimes (rarely) they may be offered for sale at the DQVC.

If you get a Stardust Sword and alchemise it as far as the Supernova Sword, you can return it to being a Stardust Sword by the alchemic recipe of Supernova Sword + Reset Stone. This is useful if you were trying for the alchemiracle Hypernova Sword, and failed, it lets you reset the weapon to the base level rather than have to find another one.

Answer 2

you can only find this weapon in blue chests in the lower floors of a high level grotto