Focus sash is not an item in Firered. It will later appear in Diamond and Pearl.
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You cant. You cant.
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.
you cant be one hit k.o.
You can find it in Silph Co. on 5F.
You cannot get the Focus Sash in Leafgreen. Though you can catch a Machoke and have small chance of obtaining a Focus Band.