It doesn't look like any of the answers given have actually answered the question you posed -- so I'll do my best:

The best place to look is within your OWN personal networks. At the end of the day, a co-founder is very similar to having a husband or wife. Your life will be tied to this person in almost every conceivable way. In fact, you may even spend twice as much time with this person than your actual husband or wife (if you have one). For that reason, it's important that the level of trust you have in this person is second-to-none.

Finding co-founders at networking events, local universities, etc is fine -- however, be sure to spend a lot of time with this person before you decide to bring them on board as a co-founder. Starting the conversation off saying "I'm looking for a co-founder" is similar to meeting a girl and saying "I'm looking for a wife."

But, again, this is why your own personal networks might be the answer. It's likely that in this group of people, you'll find people you trust who will be in your startup with you throughout the good and the bad (because, yes...there will be good and bad).

I hope this is helpful...