The problem is of course to find the voice that is best for you. The voice over portals have a filtering tool so you can find people who have ISDN or home MP3 studios, who can do kids voices or senior citizen voices, etc. However, because these sites are so large, anybody who has ever wanted to do voice over, regardless if they have talent, can sign up and clog your inbox with their bad auditions.
You can widen your choices by searching on Google or Yahoo for "voice over talent." This search will bring up top quality voice over actors’ sites. Usually the very successful voice over artists don't use the voice over portals mentioned above, but just rely on word-of-mouth and the traffic that comes to their well-ranked web sites.