In Minecraft, one may find nautilus shells from a few sources if they spend time around the sea or trade with the Wandering Trader, but their usage might not be apparent right away.
Introduced in Minecraft Java Edition version 1.13, nautilus shells still essentially only have one use as of version 1.17, and that is to be a crafting component in the creation of conduits.
When eight nautilus shells are used alongside a Heart of the Sea, a conduit can be created, which serves partially as an underwater beacon, but also as a player-buffing source capable of shocking nearby hostile mobs.
Minecraft: Where to obtain nautilus shells
Being a component of an uncommon creation in Minecraft, nautilus shells aren't the easiest thing to get a hold of. However, there are at least multiple avenues that players can pursue in order to find them.
Specifically, players can fish, defeat Drowned, or trade with the Wandering Trader in order to secure nautilus shells. Of all the options available, the Wandering Trader is probably a player's best bet, as the chances for nautilus shells to spawn in a Drowned's hand or appear as fishing treasure is relatively small.
Using an unenchanted fishing rod, Minecraft players have a 0.8 percent chance of obtaining a nautilus shell while fishing. With Luck of the Sea III enchantment on the fishing rod, the chances of pulling a nautilus shell from the water increases to 1.9 percent, which still aren't ideal odds.
Players will likely sink a lot of time into grabbing multiple shells this way, but it is considerably safer compared to battling Drowned.
If players do opt to battle Drowned, they should seek out the mobs that are holding a nautilus shell in their off-hand. Defeating these particular mobs will always ensure that a nautilus shell is dropped.
However, the chances of a Drowned spawning with a nautilus shell in its off-hand is currently 3 percent in Minecraft: Java Edition and 8 percent in Bedrock Edition. Without a committed farm to killing Drowned, searching out the right individual mobs will also be time-consuming.
The Wandering Trader NPC, often spotted in the safety of village squares but also spawns in a certain proximity to the player, offers random trades involving nautilus shells.
For the price of five Emeralds each, players can occasionally purchase the shells directly from the trader. Though it will only offer a maximum of five shells before the trade is closed, this method is still faster than fishing or fighting if the player in question has enough Emeralds.
Since it only takes eight nautilus shells in Minecraft to craft a conduit, scoring a deal with a Wandering Trader can make a huge amount of progress on that front.