The Disney autograph books are about 6 bucks.
Going along with Shells9 comment, making your own or taking something a bit different is also fun. My sister-in-law bought these really cute small (maybe 5x6 inch) spiral-bound notebooks they had at TDS for my 2 nieces... one was Pooh, the other Piglet with kind of a 'furry' feel. They were able to have each character sign their own page, then when we got home they had enough room to attach their picture with that character on the same page.
And of course, when we met the 'real' Pooh & Piglet, they were delighted to see themselves on the notebook they were autographing... my nieces still love to look at them!