Flip coins have been a fundamental part of the Pokemon Trading Card game since it began, originally they were little more than a tool to play the game but over the years they have become a collectible product in their own rights and have been released in several different sizes and lots of different designs from holo coins to matte shiney coins and so on.
The official Pokemon TCG League rule on flip coins and their usage reads as below:-
Coin flips that land outside the playing area (off of the table or in another game's playing area) are considered invalid and must be flipped again. Once players have agreed on the result of a coin flip, it cannot be redone. Once a judge has ruled that a result is conclusive, it cannot be redone.
Titan Cards offer a wide variety of Pokemon TCG flip coins spanning many expansions and series.