You will not find a better costume shop in the Phoenix area. Easley's stocks most of the things you can find online, with the added benefit of being able to browse and try them on! It's very obvious the staff loves being there and loves the products they sell.
The costumes here are a step above what you'd find at a Spirit or Party City. Expect to spend more but have a costume you can wear more than once, and it's much less likely you'll run into anyone else with the exact same costume. They literally have rooms and rooms of costumes. If you're looking for something specific, it's best to ask as you could spend a long time looking for it otherwise.
If you want accessories, Easley's is well stocked. A lot of popular accessories come in several versions from a few dollars on up. They make it very easy to buy a complete costume or assemble one of your own.
Beware that there's not a lot of breathing space in here and it gets very crowded and claustrophobic around the holidays. If you want a popular costume, go well in advance- I had to ask for several Halloween costumes before I found one that came in a size besides XS.