Bottle caps are just one of the many currencies used in the Mojave Wasteland, albeit the most commonly used, as well as the "base" form of currency used when dealing with merchants. In addition to Nuka-Cola, bottle caps can also be acquired from drinking Sunset Sarsaparilla.
Sunset Sarsaparilla bottle caps - These are the caps of bottles of Sunset Sarsaparilla. In terms of game items they are considered a separate item; however, picking up a Sunset Sarsaparilla cap simply adds to the player's total bottle cap count. These Caps can be found across the Mojave wasteland and can even be obtained by simply drinking bottles of Sunset Sarsaparilla.
Sunset Sarsaparilla star bottle caps - A rare variant of the Sunset Sarsaparilla bottle cap, featuring a blue star on the underside of the cap. These caps are part of the quest The Legend of the Star and thus cannot be sold. Once the quest is completed picking up a Sunset Sarsaparilla star bottle cap will simply add to the player's total bottle cap count.