Lunar Coins seem a random drop from eliminating enemies. They are rather rare though; it is possible to go through some runs and never ever see one drop. Get it if you do see one! Fortunately is that you do not require to use it on that run. Lunar Coins are account bound, and you will constantly have them till you choose to use them, you do not lose them when a run ends. This is very important, as the Lunar Coins have numerous uses, and you will wish to save up a minimum of 10 of them to unlock the Artificer character.