Up until two years ago, I thought that the store Party City only offered items for parties, such as themed plates, streamers, etc. So one day when I was telling my mom about the hard time I was having finding a Halloween costume for my daughter, she suggested Party City.
Party city has tons of costumes for both adults and children. They have everything from Disney character costumes, Barbie costumes, general costumes like witches and pirates, and characters like Dorothy from the Wizard of Oz.
The prices for the costumes range from $9.99 and up and the closer you get to Halloween, the better sales they will have. Along with the costumes, they also have many accessories to really make the costume great.
The sales people are very helpful. Last year when my daughter decided she wanted to be Belle from Beauty and The Beast, we bought her the costume, but then found that they didn't have the matching shoes in her size. One of the sales clerks told us we could leave our phone number with them and once they got a new shipment in, they would hold a pair of shoes for us. Sure enough, they held them for us!
Party City is a great place to shop for Halloween costumes, just be prepared that the store can get pretty busy and crowded!