Foreign Rancid Chocolate are Persian chocolates that Missy gave to Sam in iReunite With Missy, that had expired way before Sam was even born. The chocolate is also known as "The Expired 1992 Stomach Bomb," because the expiration date was in 1992, and Sam got sick from eating it. It was part of Missy's devious plan to get rid of Sam so that Missy could be Carly's one and only best friend. Sam said the chocolate was older than The Fresh Prince of Bel-Air.
Persian chocolates in the Western world
Exactly how somebody from the Western world could obtain such chocolates is unclear, since the United States and much of the Western world has had sanctions against Iran since the 1979 Iranian Revolution, partially due to their support for radical Islamists outside of the country such as Hezbollah. Considering that the expiration date was in 1992, they were either produced in Iran, during a period of attempted economic reforms that pre-dated the better known reform movement of 1997-2005, or they could've been produced by Iranian exiles. It is also possible they could've been smuggled.