Ink Sacs are the equivalent of black dye, and a crafting ingredient of Book and Quills. Here is how to get them, really easily!
Find a large body of water, like an ocean or a big lake.
There should be squids floating in it. These are the only mobs, besides guardians, who live in the ocean. They’re pretty easy to spot, so don’t worry.
Now, there are a few ways to kill squids. One way is to shoot them with a bow. The other is to swim and kill them with melee combat. Note that it might be difficult to pick up the items if they fall deep underwater.
These aren’t too important to Minecraft, but they do have their uses! Enjoy!