I'm also really curious about this, as I am a second shooter looking to work with more wedding photographers. So far I've found soem work through the local photo-services section of kijiji (but boy, some days that is wadding into murky waters), and I've done some cold-calling as well as networking through the local professional photographer's association (PPOC, equivalent to the PPA).
One thing for photographers looking to take on a second photographer, or others like me looking to be that second shooter - be very aware of the primary photographer's insurance coverage. I carry my own business insurance because contractors (ie second shooters) are not covered under insurance here. Aside from the realities of having general liability insurance in this day and age, some locations (churches, convention centers, etc) require you to be insured while working on their premise. I'm always surprised when I talk with other part-time photographers who don't bother with insurance or realize that contractually they may need it.