How to get leather from a cow without killing it? (in minecraft)?

No, the only way to get leather is to kill a cow. But what you have added in your question is what version of minecraft?

Nope

Unfortunately, the only way to get leather is to kill the cows, there's no other way. Except for finding it in chests in a dungeon.

No you have to kill the cow but you could look for a mod that allows that

Of course not! Minecraft, whilst somewhat defying the laws of physics, still follows basic logic- you can sheer sheep, make wood into planks, smelt iron, water flows down the shortest route etc…

In real life, you can't get leather from a cow without killing it. Leather is literally the skin of an animal that has been tanned and dried to make a tough material. You can't just skin an animal whilst it is alive.
Just breed your cows and kill the parent each time.