The most reliable place to see Toy Story chracters (Buzz, Woody and Jessie) is Al's Toy Barn in MGM studios. It is located near the back of the park just past Muppets 3-D. Buzz sometimes appears in Tomorrowland. Woody may appear in Frontierland, but I'm not sure (I know that he does in DL, but don't know about WDW).
Winnie the Pooh characters can be found in several places. They are all at the Crystal Palace character meal in MK. I have also seen most of them in Toontown's Judge's Tent. In MGM, over by Who Wants to Be a Millionaire Play It, there is a line of several silver trailers. Different Pooh characters usually sign autographs here throughout the day. In Epcot, I have seen Pooh and Tigger in Epcot on the characters on holiday bus and in the UK. Finally, at AK they are often along the path between the tree of life and Dino land. I have often seen Tigger in the little alleyway just past the store (sorry, don't know the name), but before the bridge. The rest of the characters are sometimes just past there, but still before the bridge. There is a path that goes off to the right, I think towards the old boat docks. There should be a sign though. I have specifically seen Tigger in all of these places.
Have a great trip!!