voiceactingalliance.com is where I usually go first for voice actors (EDIT: looks like someone beat me to it). Keep in mind certain demographics will be easier to fill than others especially depending on where you go. For example, I've typically had much better luck finding female voice actors by looking at local colleges and talking with people who attend courses in acting. It never hurts to ask! You could go to an instructor and ask if they could recommend one of their students or know if anyone would be interested. If you limit yourself to the internet, you may find that female vocalists and older male individuals will be harder to find than males age 14-30... of course my estimate of age is based on how old the person actually sounds, so I don't actually have this data lying around.
You can also try scouting voice acting demo reels on various websites such as newgrounds, YouTube, and other similar sites that may host demo reels. Try to avoid older reels (2 years or older), because usually i've found the older the reel is, the less likely the person is to respond to your email. Only go to those reels if you've exhausted your other options.
Some of the people who voice over for my series were found by actively listening and contacting people whom I felt fit the part... it isn't enough to sit around and wait for people to come to you; you have to go to them too.
Also, if your production is good enough, you can try e-mailing even people who look like they only work commercially... the results may surprise you. If they REALLY like your project they just might jump on board out of their own interest and be willing to work for free or for a reduced rate, or alternatively for a pay-for-percentage deal. One of the voice actors for the
Delura project had a small guest starring role in Star Trek Voyager... I emailed that person not expecting a response at all and got a totally unexpected reply. Sometimes commercial actors like to jump on things that align well with their interests.