Black Lens

Demon eyes are the only common source and they only fly at night, so this is a night job. The efficient version is a passive trap rather than chasing them with a sword. Lay a thin sheet of lava on top of a floor of blocks and stand in a tunnel directly underneath. Eyes drift into the lava and die, and if the layer is shallow enough it kills them without burning what they drop. Test the depth with a copper coin before you leave it running. Add a water candle and a battle potion and the spawn rate handles the rest. A blood moon or a new moon is the best night to run it.

They go into a couple of cheap early items, which is what most people use them for and then stop. Keep a few in reserve anyway, because they come back as a component in one of the summoner staffs in hardmode, and farming eyes again at that point is a slower night than it was.