First, this person may completely change their character under pressure or a feeling of impending doom, just as you need them the most. You can work with someone in the same low stress job for years and you'd still be in for a surprise when the going gets rough.
Second, you need to walk a very fine line between selling them on your idea on one hand and making sure your future prospects and goals are aligned. Selling someone a dream is easy, but it's just not enough for coping with the ongoing brutal wear that's startup work.
Third, make sure this person complements your skills with as small of an overlap as possible.
Fourth, assign responsibilities from day one, and aim for a vesting period for stock ownership (for all founders!) so if he ditches, you won't find yourself with a defunct company.
So to sum it, it's not so much about where to find a cofounder, as much as it is managing the process and expectations.
I would also hit up Meetups, Users Groups, and maybe Alumni mixers. Chances are you might run into an old co-worker/class mate that shares similar interests.
The best co-founders are people you already worked with. Go back in time, even for ex co-workers you think wouldn't be interested, meet the ones you deeply respect and start meeting with them for coffee.
Maybe they'll be interested, or at least they can recommend someone else they trust who could be. Repeat.
(Sorry if it sounds a bit negative)
