Buzz & Woody can be found daily at Al's Toy Barn in MGM.
Also the can be found sometimes in Frontierland in the MK.
As for Nemo, he is not anywhere. There is a great show with Crush called Turtle Talk in the Living Seas in Epcot. It is amazing & if your DC likes Nemo, the show is a must (it is a must for anyone I think though!).
But since Nemo is a fish, you cannot find him out of water.
Actually, there is a Nemo "character" you can have your picture with outside of the Living Seas building. He's a bit different from the other characters but he is there.
Definately ask a CM near the Living Seas. I'm not sure how "announced" his appearances are beforehand but we also saw him last Dec. So I know he exists!
Nemo - definately the area in front of the Living Seas at Epcot. Not sure how often he appears, though. Like most characters, he does 20ish minute meets on and off during the day.
Buzz and Woody are together at Al's Toy Barn, near the back area of MGM Studios. They are very consistant there, making daily appearances. Jessie may be there, too!
At Magic Kingdom, Woody and Jessie make appearances either in front of the Diamond Horseshoe building in Frontierland, or inside. Sometimes Bullseye is there, but he is "rarer" than the others. Buzz also makes appearances in Tomorrowland, usually near Timekeeper, right off the main sidewalk. We seen Buzz in other spots in Tomorrowland, too.