So, you've played KH and now you want more bosses, enemies and keychains? Then look no further than KH: Final Mix!

KHFM is an upgraded version of KH. With added bosses, new characters, new items and much more! KHFM is currently only available in Japan, and is not available or due for release in the USA or the UK. A lot of people buy this game on import. It can be quite pricey, but it is also worth it. Merchandise of the game was released with it, and they can be found in the Merchandise section.
Final Mix is certainly worth the money. You can also buy a cheaper 'Ultimate Hits' collection version as well. Exactly the same, but not the original cover. Well. Sort of. No Kingdom Hearts collection is complete without it though.

The original edition will cost you about $69.99, and the Ultimate Hits version will cost you about $33.00.