Try looking for a Best Buy Express vending machine inside YYZ. They just might have an iPhone 6S available. The pricing shouldn't be too different as I remember the pricing on my Bose noise-cancelling headphones was pretty competitive (or even the same) with retail shops in the city.

There's also a chain of shops at YYZ called iStore that I remember did have Apple products. I think that 6 hours would be a bit tight of a layover with traffic and everything. However, you will have to clear Canadian customs anyways.