It's been a long time since I worked with an actor I didn't already know, so I apologize in advance if this method proves outdated. That said, it used to be that talent agencies had CD compilations of their voice-over talent, which can help a lot in narrowing down the field. (SAG can provide a list of agencies. The commercial or voice-over division of a given agency is the one you'll want to contact.) Depending on the talent, their level of interest in the project, and your ability to negotiate with their representation, you can sometimes get voice-over talent for well below scale. The trick is to get around the agent. You might try writing a letter (sent to the agent, but addressed to the actor) that really talks up your project and states in no uncertain terms how vital the actor's participation is to its success, but does not mention pay. Try to get him/her interested first. In my experience, if an actor wants to do something, he does it, regardless of whether or not his agent thinks it's a good idea. Say in the letter (in a p.s., so it's the last thing he reads) that you'll be following up with his agent, and then do that. With any luck, the actor will have told his agent he wants in on the project and the money will be less of an issue.
All that said, if you're only planning to use the actor for scratch, you may be facing an uphill battle – when an actor forgoes money, it's usually in exchange for exposure. If you're not offering exposure, you'll definitely want to focus your energies on smaller agencies and lesser-known talent.