At the Magic Kingdom you can get them at the Fry Cart in Frontierland...it's down the path from Splash Mountain toward Pirates of the Caribbean.
At EPCOT I believe you can get them at the small refreshment stand on the Canada-side of the entrance to World Showcase...across from the dock where boats take you to Japan/Morocco.
At the Studios you can get them on Sunset Blvd. on the same side of the street as Rock N' Roller Coaster, but closer to the Villains Shop.
At Animal Kingdom you can get them at Restrauntasaurus in Dinoland and at a refreshment stand in the Asia section (on the opposite side of the path from the Kali River Rapids ride entrance.
Hope that helps. You'd think I was a McD fry-aholic wouldn't you? Actually I do love them but have never gotten them in the parks. I've seen others with them and they look great!