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Like many other low-cost air carriers, you cannot search flights or book tickets on NewLeaf through online flight search engines or booking sites.
NewLeaf have this to say on their About Us page:
Remember www.GoNewLeaf.ca because weβre not using any booking sites. To keep costs down, we sell directly to the customer and a small number of exclusive partners.
(Those partners are not named, and that information doesn't appear to be available yet.)