I haven't found a bullet-proof method, I'm afraid. The large majority of artists I've contacted have turned out not to be suitable, too expensive, or (mostly) just flaky. My primary methods have been browsing deviantart in relevant categories, and finding active artists with either the same art style that I want, or a show of competence in multiple styles; or the same evaluation but applied to the Indie Gamer art portfolio forum.

One thing to bear in mind is that even if an artist is too busy to work for you themself, they may know people who they'll be happy to refer to you.