The Crescent Moon amulet is a unique item, so it will only drop randomly. You can't be sure when or where you'll find it. However, in the game, there are Diablo 2 Resurrected Items for Sale certain areas where they have a higher chance of falling, so you have a higher chance of finding them.

Typically, you'll most likely get a Crescent Moon amulet at the end of a game on Nightmare difficulty, or at the start of a game on Hell difficulty. Andariel on Hell is the most likely monster to drop the Crescent Moon amulet, followed by Mephisto and Diablo on Nightmare, and Baal on Normal. In Normal mode, Baal and Nihlathak are the only bosses that can drop the Crescent Moon amulet. And normal monsters in Normal mode don't drop one at all. Even before Act V in Nightmare, the normal monsters in it didn't drop one, and normal monsters other than that were extremely difficult to Buy Diablo 2 Resurrected Items drop. So either you choose to keep grinding the boss, or you choose to play the first few Acts with less repetition on Hell difficulty.

Once you find a Crescent Moon amulet, this item will be very useful to you. It allows you to drain life and mana at the same time, and if you pair it with the Sorceress' Energy Shield skill, they are very powerful.

In general it is very difficult to farm a Crescent Moon amulet in Diablo 2 Resurrected as its drop rate in the game is generally very low. So if you want to get a Crescent Moon amulet faster, I suggest you check out MMOWTS. Their shop sells a variety of Diablo 2 Resurrected Items, which will also contain the Crescent Moon amulet you need. You just need to place an order now and use them in the game as soon as possible.