The simplest way of getting Enderman heads is by killing Endermen. Chance of getting a head is 1/40 (2.5%), which is the same chance as the chance for a Wither Skeleton to drop his head, but because Endermen are found across the whole End dimension in large numbers, their heads are considered to be less rare. Each level of Looting enchantment also increases the chance by ~0.065%.
Another way of acquiring the head is by exploring a Dungeon Tower. Even though each floor of the tower is randomly picked, you will most likely get at least one floor with 2 or more heads.