Pottery Barn Kids is having a Premier Day sale today and Halloween costumes are included. You can get 30-50% off costumes and shipping is free.
PB Kids costumes tend to be a little pricey but they are always such good quality.
My youngest was a fairy a few years ago and it was a really lovely costume in person.
You can click here to see the sale. Please note I believe this sale is running 9/28/19 only.