I've purchased autograph books both ways. In the park, they are pricey and you just might run into a character before you have time to get said autograph book. However, if you must have the
Disneyland logo on the autograph book, then I'd suggest buying it there.
For my next trip, I'm getting ds (3.5 years old) a really cute personalized autograph book/bag/pen set done in a Pluto theme from someone off Ebay. It should arrive in a few days so I'm excited to see it.