Williams has been around here for-EV-er. As a participant in a lot of theatre, I've been visiting them for almost as long.
They have a large selection of costumes to buy and higher quality costumes you can rent. They have Halloween-type makeup as well as professional stage makeup. The people who work there are very knowledgeable and will help you as much as they can if you have a special request. There's also a selection of fabrics in case you're handy with the ol' needle.
It can be pricey to outfit someone here, but sometimes that's easier than pulling out the ol' sewin' machine.