Resveratrol is one that has been studied. At first this compound was credited with the health benefits of red wine. However, new measurements show there is really very little resveratrol in red wine or anywhere else. So this class of compounds is not yet very important for human diets. Lignans are usually classified as dietary fiber.
Flax seed is one dietary source.
Foods and beverages each contribute about half of the polyphenols we consume although the proportions can differ greatly depending on individual food choices. Studies of intake and of food composition are hampered by inadequate methodology. We lack suitable analytical methods to detect some of the phenolic compounds. Just because polyphenols are present in foods does not automatically mean they are well absorbed. Studies of bioavailability are underway.