You have to win it in the sphere break contest in Luca,in Chapter 3. If you don't succeed, Shinra will win it and refuse to give it to you, you'll have to play him later for it.
Yeah... Im sorry to say that your gonna have to beat shinra at sphere break. It should be easy enough if you do lots of digging in the central expanse youll find lots of rare coins... Good Luck!
