From my experience the best opportunities come through people you already know. That's why i make a effort to get out there and meet smart/interesting people who are likely working on one or more projects at the same time, and will create new opportunities in the future. If you leave a good impression they will contact you when something comes up.
However, startup events, mixers, and meetups are always packed with business guys looking for a tech cofounder. Naturally, the low-hanging is fruit not always the freshest. In other words, the quality may not be the best but you're sure to meet people wanting a tech co-founder. You'll also meet a lot of people with "Facebook for pets" style ideas, and people who underestimate the degree of technical complexity in their projects.
I've also gone to talks at business schools, and there are always young people looking to start businesses there as well.
- meet lots of people.
- keep in touch with the good/interesting ones.