I just finished recording a project in which I collaborated with a few vocalists. Based on that, I have a few recommendations (and you may already know this... if so, then maybe it will help someone else reading this thread):
1. Don't know that you'll get much from a producer's site. Go to soundcloud and linkedin to find what you're looking for. There are a ton of vocalists (and writers) that are open to collaboration opportunities.
2. If you do go to soundcloud and linkedin, don't use the word "demo". It immediately makes people think "limited potential outcome from my effort", and you probably won't get a great response. Very few people want to commit their time to something that doesn't have decent payoff (monetary or exposure). I would use "song", "collaboration", "co-writing", etc - even if you only intend to use it as a demo.
3. My perspective is that even for a "demo", the vocalist can make or break the perception of the song, so getting a decent vocalist is essential. If you're not planning to pay this person for their time and effort, you probably won't get a decent response unless you offer something. Typically, I have a handful of tracks to offer in exchange... "hey, if you record this song for me, I'll produce one of these tracks for you... pick one of them, and it's yours". No money exchanges, everyone "wins".
4. I could certainly recommend a few vocalists that may be able to help you, but no way I'm putting my reputation on the line making that introduction without hearing your music. lol My peeps trust me - if I recommend a dud to them, my rep suffers.

Include a link to your music in your post dude!
Again, you may know these things already, and perhaps you have a nice rep on here already and I just don't know who you are. Like I said, maybe this will help someone reading this, if it isn't helpful to you.