What I did is I post auditions in two sites:

http://voiceacting.proboards.com/index.cgi

http://voiceactingalliance.com/board/forum.php

Most of the details are in the stickies on how to post an audition. Let me give you additional tips:

Since this is going to be a web series, make sure you let them know of this because this is not a one shot thing and requires some commitment in their part and post a very descriptive background or summary of what your project is about and importantly, post artwork or screenshots of the characters that are open for audition. The more impressive they look, the more they will be motivated to join you.

Give it at least a week to audition and take your pick ONLY after the deadline so that it will be fair for them.

Most of these people are young adults and some are actually professionals if you manage to attract them. Expect some delays and some of them suddenly getting sick and full of excuses for not being able to send their pieces in time.

Encourage them, complement them and be patient with them. Try not to be angry because it is important to make good relationships with these people especially the older, veterans who will be more available to you if you decide to make another project.

Be more demanding only if you have money to pay them.

Passion is anger and love combined. So if it looks angry, give it some love!