I wanted to jump in here and to caution that finding a cofounder with the specific intent of applying for this round of YC would be a bad idea. Keep in mind that a co-founding relationship is very much a relationship, and you have to "date" a bit before you get married — having a startup is like having a child, you want to know a potential cofounder is the right person to found a startup with first.

That said, be inspired to try finding a cofounder now, work with them for a few months and keep S13 in mind — it might all fall into place by the time the next round comes around. And if you can't find a cofounder, don't be afraid of being persistant and just learning the skills necessary to build and launch startup solo, since a great cofounder is likely to come along if you already have an intriguing product launched.