Clubs +2 are only available from the Weapon Merchant in The Black Pits II: Gladiators of Thay.
In Baldur's Gate II: Enhanced Edition there's a unique club +2 named Mighty Oak, other than its name however, it has no meaningful difference from a generic +2 club.
Description[]
| “ | Most clubs are stout, hardwood sticks, narrow at the grip and wider at the end. This simple weapon has been used since people first began using tools. Anyone can find a good stout piece of wood and swing it; hence the club's widespread use. This particular club has been magically enhanced. | ” |