Where could I buy the notebook that looks like the real Death Note?
Check out these links:

http://www.399animeshop.com/anime/death-note/death-note-book-replica-size.php
http://www.ebay.com/bhp/death-note-notebook

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.