I got each of my kids a small square black and white journal from Walmart for about a dollar or less each. They are about 3 x 4 inches or so. I attached a photo of each of them to the front with their favorite disney charactor from a previous visit and put a piece of 2 inch clear tape over it. The best part is they fit very well in either their pocket or the front pocket of the book bag for quick and easy retrieval. Sometimes you run into an opportunity and have to get those books out fast.
Also since the pages are lined, when we get home I label on the back which park, resort, meal date ect.... So they will remember where they got each signiture, and if it is the safari mickey, or the MGM paint costume Goofey or whatever.
The charactors always make such a fuss over their special books, my 15 year old neice told me she didn't want a regular book, but one like my kids had. I ended up making one for her too.
Sam