The first time Margaret Dron organized the Gluten Free Expo early last year, it was inside the gymnasium of a small community centre in east Vancouver. She had recruited one volunteer, two speakers, 38 vendors and expected 500 attendees. There was no entrance fee—instead, people were to bring gluten-free goods for the local food bank; three boxes were set aside for the collection. Six hours later, more than 3,000 people had turned out, and the volunteer had to call a one-tonne truck to pick up the donations. In one Sunday afternoon, Dron realized, “there is some serious potential here. So...