Smelt cactus in a furnace, blast furnace, or smoker to get green dye
Find green dye in desert village chests
Find green dye in shipwreck supply chests
Find green dye in desert temple chests
Trade with wandering traders when they offer green dye
Use green dye on white sheep, wool, glass, terracotta, beds, candles, banners, shulker boxes, or other dyeable items