particularly Heroic dungeons

The fastest way to earn Justice Points is by running dungeons (particularly Heroic dungeons),

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 but they can be earned through Scenarios and even converted from Honor points if you prefer (though you’re probably better off just buying the Honor gear . . . more on that later). Once you’ve got some Justice Points saved up, head off to the quartermaster at Niuzao Temple in Townlong Steppes to get some sweet, sweet epics.
Valor Points still have their place as well. For just 250 Valor points, you can add four Item Levels to a piece of Epic gear, and you can upgrade any piece twice.

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If you’re looking to gear up in a hurry, that’ll probably give you the most bang for your buck. Even if you don’t have anything you’d like to upgrade just yet, it’s likely more worthwhile to hang onto your Valor until you do.