I keep reading where people want to take pre-dyed leather and redye it. PLEASE UNDERSTAND THIS: Unless the piece is vegetable tanned and has not had and type of finish applied to it, you will not be able to get any type of dye to stick to it (unless something brand new has hit the market). You might think you got a good dye until everything you are wearing gets dye on it.
This is the reason that this costume is difficult. You can spray dye leather (which is like painting it), you can paint leather, but you cannot redye finished leather. You may be able to strip veg tanned leather, but the chemicals are very dangerous and it is not worth getting sick over.
Leather is expensive. I'm just trying to help you.