While they erroneously left hiring Yours Truly off the list (ahem), this article does contain some excellent tips for businesses looking to hire voice talent for their projects.

Additionally, Paul Strikwerda adds comments addressing the real value of hiring a genuine VO pro; it's required reading along with the main piece. (To paraphrase Paul here: to suppose that having a nice voice can make one a voice actor is to suppose that having an attractive pair of hands qualifies one to be a concert pianist.)