The Thunderhammer Smithy sells this crossbow. It's one of the most powerful crossbows in the game (along with The Guide, which adds more damage and THAC0 to projectiles, but has the normal rate of fire). Like many powerful weapons, it is also quite expensive.
Baldur's Gate II: Shadows of AmnEdit
Bernard in the Copper Coronet sells the weapon. However, in the Underdark, there are Drow Crossbows of Speed available. They are +3 weapons and have better stats, but they will turn into adamantine dust when taken out of the Underdark.
|“||Light crossbow of speed: 'The Army Scythe'. A short-lived human outpost in the Frozen Forest unearthed this weapon, and many others, from the ruins of an ancient settlement deep within that cold land. Never knowing the names of their extinct benefactors, the colonists used these "tools" to carve a large territory for themselves, though in the end it was for naught. Crushed by a superior number of hobgoblins, these unfortunate souls have joined the ancients in their anonymity.||”|