Bolts of Polymorphing are rare, expensive, and occasionally dropped as treasure (in single units). The bolt is available in Baldur's Gate: Enhanced Edition as well as Baldur's Gate II: Enhanced Edition. In the BG:EE, a few may be found for sale apparently at random sold in single unit increments (at Ulgoth's Beard Inn, Thalantyr the Conjurer's home, and the very high end shops in Baldur's Gate such as Sorcerous Sundries).
The bolt strikes for standard 1d8 piercing damage as any other crossbow bolt and forces a save vs petrify/polymorph. If the target fails, it is transformed into a field critter (such as a Squirrel.)