On Craigslist. I put an ad up about my current company and that I was looking for a cofounder. I was contacted by two people. I contacted the first, but he never messaged me back. I corresponded with the second for a bit, and after a few emails we realized we had met before at a hacker party called Super Happy Dev House. We met for lunch, and the rest is history.
You should come to the next Devhouse http://superhappydevhouse.org/SuperHappyDevHouse21. It's on Nov 10th in SF.