Well... I just saw the notebook in my fellow schoolmate and he just brought it to school yesterday. And I'm really curious where he got that and how much did it cost.
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There are different types tho, one type with only the cover looking like the death note, the content is like a normal notebook. The other type is exact replica, figure out which one you want.
Another way to get one might be going to a convention (gaming or anime). I don't know where you're from but I got my Death Note replica at the GamesCom (Germany). As Death Note is a really popular anime, you'll find a lot of it on conventions, not only the notebook itself.
So my guess would be, that you would be best off going to your favourite online webshop and search for death note notebook and you will find what you were looking for.