You will most likely get this during your playthrough anyway. You simply have to defeat 7 infamous pirates, i.e. the ones carrying the pieces of eight. Most will declare war on you and you will cross them on the ocean anyway, so you won't have to go out of your way to defeat them. Some are a bit strong in the beginning, but by the end of your midgame, you should be able to 1-on-1 each of them.