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.
Check out these links:
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.
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.