It's regulary dropped by drow warriors who can be found all over the Underdark.
|“|| Drow Longsword +3
These swords are usually referred to as doubled-edged swords, war swords, or military swords. In many cases, the long sword has a single-edged blade. Most long swords have a double-edged blade and a sharp point at the tip. Despite the tip, the long sword is designed for slashing, not thrusting. This item, like most drow equipment, is created using Adamantine, an alloy of adamantite that quickly turns to dust if removed from the Underdark.