A Flagship Battle is a One Piece TCG tournament run in stores. At the end, two cards change hands: the Winner, handed to whoever takes the store final, and the Best 8, given to the eight semi-finalists. No one can buy them in a booster, you have to win to pull them.
Here is the catalogue of every series, with the Winner and Best 8 card of each. For the detailed, browsable version, series by series, head over to the Flagship hub.
Japan and Asia: two separate calendars
The programme runs separately in Japan and on the Asian circuit. The Asia series reuse the same artwork as Japan, but with a 12 to 18 month lag and different print runs. For any given card, the Japanese version has so far stayed the more expensive one, because it is rarer.
Going further
If you are new to official tournaments, start with how the One Piece TCG circuits work, it explains where these cards come from, and why you can’t buy them.