Do you mean where specifically, or which one? That set is in various chests throughout Stone's flying fortress (teleporter access on the north end of the island in the channel on the way to the harbor).

Specifically (from the walkthrough);
-The ten piece Archmage set is hidden in treasure chests around the fortress.

1: The ring is straight northeast of the entrance, right next to the first
generator.

2: The cuirass is slightly northeast of the last (northernmost) generator.

3: The gauntlets are just a couple tents south of the cuirass.

4: The earrings are on the same platform as the gauntlets and cuirass, at its
southeastern end.

5: The belt is on the same platform as the gauntlets, cuirass, and earrings,
but one level up. It's just south of the ramp leading up to the top level.

6: The leggings are on the same platform as the last several pieces, but at
the very top, on the northwest side.

7: The helmet is hidden on a tiny platform south of Stone's Greenery.

8: The necklace is on the east side of a small platform overlooking Stone's
Greenery from the north.

9: The bracelet is on the northeastern side of the lower tier of the large
platform beneath Stone's Greenery.

10: The warhammer is northwest of the entrance, two levels up.