Visit Calbridge Stronghold, then find Hiroku. He will suggest you to buy special "Gambling coins" from him and play his game. Buy said coins without any hesitation and gamble! Don't forget that in order to get your reward you must talk to him. Also, no matter if you win or lose your gambling coin is wasted, so you need to buy another one. Also, just to speed things up always select "cup A" (it doesn't matter if you constantly pick the same cup or different cup, but "cup A" is selected by default, so you just need to press Enter). Mathematically speaking odds are in your favor, so on average you will get richer by gambling: -250+(1/3)*0+(1/3)*0+(1/3)*1500=250 (this is the expected value).
Here is map:
Exact place where you will find him:
