If you havent tried it, check out http://startupers.com/
You can see others have posted ads for co-founders etc..
Anyway these are just tools to find people. But more important than the tool is how you are going to know the right candidate when you meet them... I dont claim to know the answer to this.
They have to be so passionate about your idea that they should make it theirs.
They have to wake up in the morning thinking about this first and then everything else.
They have to be willing to roll up sleeves and do whatever it takes.
The two of you have to gel... really well
Unfortunately, it is difficult to know all of this from a few conversations. SO make sure you bring them in with a 1 or 2 month trial for both of you. Be open and say you want to see if you two will work out.