Balloons have been a part of Disney theme parks since forever. World Bazaar usually has several vendors out at one time, selling different types of balloons.
When I was a kid Mickey Mouse balloons were made of rubber and barely lasted 24 hours before the helium escaped and the balloons would no longer float. Today they are made from Mylar or other materials which hold the helium longer. They also come in a wider variety of shapes than ever before, and feature not just Mickey Mouse but many of the major Disney characters.