Here's a link to a party store selling the costume pretty inexpensively after Halloween. It has a small battery pack for 4-AAs that hooks on your pants and connects to a small fan on the side of the costume that inflates it when you tighten a waist line draw string. Worked great and probably available at a number of different places.
www.partybell.com/p-50055-big-animal-head-costume-shark-with-legs-adult.aspx
