Buzz and Woody and Jessie popped up outside Al's Toy Barn at 10:30 one morning. (They were scheduled almost every morning, from what I could tell...) There were only about five or six people in line, and I got my picture snapped, pronto. So if you REALLY want a shot, get there a few minutes early and you shouldn't have a problem.
Likewise, after that little adventure, I walked a few feet away and saw Miss Piggy posing outside the Muppet Theater. I had my picture taken with her, too! (I am SUCH a character geek...)
Meanwhile, Mike and Sully are, as mentioned, right across from the ABC Commissary, in their own little nook, at various times throughout the day. A Disney photographer will snap a picture that you can pick up and buy later near the park entrance. I don't know if they let you snap your own shots, though.