Where do you get the strongest sword in Dragon Quest IX?

Answer 1

There arguably is not a 'strongest' sword in DQ9, so I will list the two contenders.

The sword with the highest attack power is the Hypernova Sword, which requires an alchemiracle. The starting point is the Stardust Sword, which must be found in high level grottoes, or if you are lucky, bought from the DQVC. Stardust sword, alchemised with one Agate of Evolution and one Silver Orb will give a Nebula Sword. Nebula Sword, plus 3 Agate of Evolution, and 3 Silver Orbs, will usually give Supernova Sword, but may produce a Hypernova Sword.

However, for any character with a strength higher than 140, an Uber Falcon Blade will do more damage than if that character were to use the Hypernova Sword, due to striking twice. That counts even more so when Falcon Slash is used. A Falcon Blade can be found on your ship, and bought from the shops in Stornway after you complete the main quest. You will need to alchemise it with a Meteorite Bracelet.