If you are not interested in buying a Halloween costume this year probably due to budgetary constraints maybe you can make your own. Here are some links to help make make a wonder woman costume for about 10$. There are some great step-by-step instructions for a wonder woman headband template here which is difficult to find for some reason.
Wonder Woman Headband Template
You if you do not feel like making it you have a couple cheap options to buy
Option2) You can get a temporary headband tattoo Wonder Woman Crown Tattoo