You can get them in supplements or in your food. Best absorption is probably with food, best control over dosage is with supplements.
Food sources: EGCG, nothing better than Green Tea. I make ice cubes out of Green Tea every day, and put 'em in every, every, everything I drink. You can also use Green Tea powder (Matcha). Other teas, like regular Lipton etc., to a certain extent.
Food sources for Quercetin: #1 way above all others is capers (yes, those tiny salty sour green bud things) or caper-BERRIES, same plant but the berries rather than the buds. You only need a bit. Three tiny capers used as condiment for one smoked oyster (for zinc) on crackers, and you've practically got it made.
Many other sources of quercetin, but you'd have to eat greater volumes: onions and all members of the allium family (garlic, shallots, chives, leeks).
Most deep-purple berries (elderberries par excellence, also blackberries, blueberries, deep blue grapes and juice.)
Apple (skins), kale, cocoa powder!
Thank you for that fascinating information. Are you saying that three capers on a single oyster from a can of smoked oysters will give you protection from Covid19?
I happen to like both oysters and capers.