Deciding to make someone a co-founder is a huge decision. You are starting a business with this person, and I suggest you really get to know someone before starting a business with them. Its easy to start a business, but its very difficult to see one through to success. The path is not easy, and although you can use something like the Github search to find candidates, I suggest you spend time working with someone before you go and start a business with them.
I'm seriously working on the co-founder matching problem (have been for several years now). I believe in a mix of offline (google "co-founders meetup" for a location near you), and online (my startup, http://foundrs.com will introduce another really new and innovative feature for co-founder matching hopefully soon - contact me if you want to be a guinea pig).
