The difference is that Focus Sash works when your Pokemon would have been KO'd in one hit from full HP, while Focus Band has a 10% chance of protecting your Pokemon from being KO'd from any HP.
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You cannot get the Focus Sash in Leafgreen. Though you can catch a Machoke and have small chance of obtaining a Focus Band.
Focus sash is not an item in Firered. It will later appear in Diamond and Pearl.
no way to get it
You cant. You cant.
you cant be one hit k.o.