There's bulletin boards at the Balboa
Yacht Club and Shelter Bay Marina where lots of people (cruisers, backpackers, locals, etc) put their contact details on for crew through the canal. Mix of volunteer, reimbursement for travel back from the other side to full paid. Fully paid is mainly the locals who do it regularly.
I think you'll find very few who are looking at forums
or on-line bulletin boards for crew. It's generally easier dealing with people on the ground when you're there as scheduling can be fluid at times.