In Baldur's Gate, it's sold at the Shop of Silence and Black Lily will sell one, but it sells for more gold than the other. You may also earn one as a gift from a farmer in the area north of the Friendly Arm Inn after helping farmer Brun. Wilton, who is inside the Fishermen's home, also drops it if killed. One cloak can also be found in the Nemphre's house in Baldur's Gate SE (at least in the Enchanced Edition).
In Baldur's Gate II: Throne of Bhaal, it can be upgraded by Cespenar to Improved Cloak of Protection, which requires one scroll of Improved Invisibility and a scroll of Improved Haste and a sum of gold.
|“||This simple cloak has been imbued with some small magical properties to help protect its wearer. It does so by affecting both saving throws and armor class. It is ideal for mages and others who cannot wear bulkier or more restrictive types of armor.||”|