If you think about it, any color could work, because if you have read all the manga, and everything and never seen or found reference to the color of the note book, you can't be wrong with any color you do. Because if you don't know the color how is any one else going to?
I'd suggest a light color, like a tan or as Dustin said, red or brown. Though it can be any color, I am sure you wouldnt go with black or white because of the shading in the manga scan. That way the writting you will be doing on the cover is easily noticable, as for somewhere to get a note book of that size, try... an art store, micheals perhaps? I don't know if they sell art books there, any large art or craft store should have a variety of note book sizes, the only thing to worry about is they can get quite pricey and the binding may not be the way you want it or may be on the wrong side for the size of note book you want.
