I don't know about sites where you can actually get ahold of voice actors for something like that, but there's a few people on YouTube who are usually willing to do voices for nothing.
I can recommend a great friend of mine, Thomas Ferranti (
http://www.youtube.com/RevSpike), he's a pretty good character and voice actor, he has a repertoire of different voices, he recently did a voice for me and did a great job.
I'm pretty sure he has a demo reel on his YT channel, you may want to check that out.