In addition to what others have said, one other avenue is to go to local startup/entrepreneurial/tech events and meetups in your area. I've met my co-founder at

In addition to what others have said, one other avenue is to go to local startup/entrepreneurial/tech events and meetups in your area. I've met my co-founder at a local startup-related meetup. And I know some people that were introduced to their current co-founders through people that they met during such events/meetups.