Hi Candace,
We love Mickey's Not So Scary Halloween Party! Some of the costumes that guests come up with are amazing and I love that Mickey and his friends get into the spirit and dress up too.
We have always brought our costumes with us in our suitcases. There is a great selection to choose from at
www.disneystore.com or even our local Disney store in the mall. We also have a large Party Store in our town that we visit for our costumes. If you are looking to purchase a costume once you arrive at Disney, the best place to visit would be the World of Disney at the Marketplace. You will find the largest selection of Disney merchandise, including costumes, here.
You are welcome to bring your own bucket or trick or treat bag with you to the party, however each guest does receive an empty treat bag once you enter the park. It typically is a small size plastic bag (think small gift bag size) with handles. The bag usually gets quite heavy and full so we often dump it into our backpack throughout the night to make it easier for the boys to carry.
Hope you have a boo-tiful time!