You can always count on Halloween Express in finding the perfect Halloween costume. The selection is HUGE. There are dressing rooms available to try on the different costumes to make sure you're really getting what you want. The employees are always dressed up in a costume and are very helpful if you have any questions--and are great in helping you make your decision.
One hint that I've found is if you wait until the final week of Halloween, specifically the last few days before, you can really find a great deal because everything gets marked on sale! I usually find items to put away and use for next year.