If you don't already have a good co-founder, perhaps you are not yet ready to start a company. Try joining an existing startup instead. If it succeeds, that's great, but either way you've learned more about startups and hopefully met some people who would make good co-founders.
If the startup fails then you'll have been working with other passionate people who are likely to be willing to give it another try. Perfect for cofounders.
There are online cofounder finder services available such as http://www.buildv1.com/ but I have yet to hear of any succesful partnership coming out of them. Best to stick with people you know in the real world it would seem.
