OTTAWA—Ticked-off Canadians, irked by U.S. metals tariffs and President Trump’s harsh words for their prime minister, are boycotting American products and buying Canadian.

“Usually we don’t pay that much attention to it,” said Garland Coulson, an entrepreneur from Spruce Grove, Alberta. “You tend to buy the products that taste good or you buy the products that are low in price where taste isn’t an issue.”

But...