This is really easy to do once you gain access to Chapter 5 as you gain access to all your items collected throughout the game, and can then sell them off as Akiyama.
However if like me you want to get this during Chapter 1, then I found that playing golf was by far the most lucrative method. Winning each tournament nets you just under 100,000 yen, so should only take a little while. It's also a nice way of building up a decent amount at the start of the game for hostesses or gambling.
Otherwise I would recommend just fighting random enemies as they can often gift you a reasonable amount, or if you are really lucky, a gold/platinum plate.