Craigslist has worked for me in the past. A sous is someone I usually like to promote from the line, there are no suprises for them, they've seen the job being done and usually have a really good idea what I expect from them. Your sous chef is the one person in the kitchen you can trust, the one who will fill your shoes and have unwavering loyalty. Of course that is a best case senario but you'll have a better chance if you know a little about how this person operates before hand. Unless someone comes highly reccomended from another chef, they usually will start out on the line and move up from there. Plus promoting from within sets an example for the others in the kitchen to follow: do a great job and be rewarded.
If this is a new place building a team from scratch, you'll be in the same position as I unfortunately...screwed. Looks like for the first month or so I will be exec and sous (and pastry) until the picture of who we've got becomes focused. My hope is that waiting a little bit will be easier that making a premature judgement and having to demote or fire someone later on. For me, I really need to have a firm grasp on who my sous is and how they work before I put my *** on the line for them. Remember, all sous chefs think they do all the work and get no credit. This is not true. I have never treated my any of my sous like a scapegoat or made one to be a workhorse that carries my load. This will just create resentment and problems. In our restaurant, we train ALL of our employees to treat the sous like me. Just as much their boss, just as capable cooking and leading as I am (and sometimes better) It's like when you vote for president, you never want to look at the vice president and be afraid he will ascend to the throne. He or she should be just as capable as the chef with the only difference being I get the final say in things (and a little more money) lol