There's a couple of ways to do this.
The easiest and best option is buying one of Indy Magnoli's. His is the diary against which all others are judged. They are a bit pricey (does anyone know what they're going for exactly nowadays?) but well worth it.
Then there are the do it yourself ways (the route I'm taking).
There are a few websites who have the diary available in printable form. This means that you have to print it out and then fold and sew and bound your own diary. Yyou have to get leather for the cover and figure out how to put a book together (a lot of work but very rewarding)
Then there's the option that I'm taking. I purchased a close enough diary from here
http://www.jerrysartarama.com/discount-art.../art-supplies/5 (4.75"x6.5" - I forget whether it's dark brown or medium brown), Then I downloaded the diary text and now I'm hand writing it. It takes a lot of time, but I think it offers a more realistic feel. Some of the drawings are done freehand, others are printed out and then I use carbon paper to get them in the book and then go over it with my pen.
I believe (not entirely sure) that magnoli also offers a blank grail diary, so you could, theoretically, combine two of the methods. You'd have the exactness of the physical diary, but also the realism of having it actually be hand written.
Does anyone know for sure if he offers the blank ones and how to go about getting one?
The choice is yours, but choose wisely... (sorry, couldn't resist)
-Fred