D.M.
Have you heard of FreeCycle? It's a network of people who post things they don't want anymore or ask for things (like this) that they need. The person receiving picks up from the person offering. Only free items.
The network is organized into separate Yahoo Groups by community. Do a google search on FreeCycle and your community (or a neighboring community) and then request to join the group. I bet you would be able to put together the costume in a couple of days. I've been amazed at how much traffic there is on the website and what types of things people are exchanging. It's a great network.
Good luck,
D.