Short of EGCG capsules, the best source is matcha, but good quality, not the stuff being hawked in coffee shops.
And EGCG seems to work for a number of cancers. Back when I was there, it was a paradox that twice the percentage of Japanese smoked as Americans, but less than half the percentage of Japanese got lung cancer, and the presumptive reason was the daily drinking of either brewed or whisked green tea.
My opinion is obesity encourages cancer cell growth with all the extra nutrition floating in blood. Japanese people in Japan are less obese than Americans living in America. I avoid green tea because it acts as a diuretic in my body. Zero cancers, zero heart issues at age 85 by keeping my weight under 154 lbs with daily exercise and avoid eating more than body needs.