Where are the final weapon orbs in Secret of mana?

Answer 1

For the sword, you don't actually get a final weapon orb. You have to cast both of Dryad's Mana Magic spells from the girl and the sprite onto the boy. This will termporarily give a huge boost to his attack and turn the sword into the Mana Sword. There is no permanent way to gain this, however, without glitching your game. I would not recomend this because it corrupts save data.

 

However, for the rest of the weapons.. The final weapon orbs drop from monsters inside the Mana Fortress. These are rare drops, so they take a bit of grinding, but if you can keep farming them until the desired weapon is Level 8. This is great if you happened to accidentally miss some of the orbs. Here's a list of drops:

Gloves - Wolf Lord

Axe - Fiend Head

Spear - Tsunami

Whip - Terminator

Bow - Metal Crab

Boomerang - Master Ninja

Javelin - Whimper

Answer 2

The 9th version of the Sword is the Mana Sword. The only way to get this during "normal" gameplay is via Dryad's final spell, "Mana Magic". When both the Girl and the Sprite cast "Mana Magic" on the Boy, the Dragon Buster becomes the Mana Sword.