I am not experienced in the costume world by any means. However, after being dragged through the crappy seasonal "Halloween" stores, and digging through the all-too-picked-over Good Will in preparations for Halloween, I was extremely surprised and pleased with Spotlight Costumes. I have walked by the place thousands of times, and always thought it was some kind of sleazy second rate kinky-costume store.
The store is tight- because it is packed with great costume pieces. Their large variety includes fake mustaches and hair pieces, all sorts of masks, and full costume sets. The great thing seems to be that, as they don't appear to rely solely on the local Halloween crowd (Their website identifies theatrical and retail rentals as well) to cover their expenses, they are able to provide a large variety of costumes at reasonable prices. A breath of fresh air from the bulk of Halloween stores in the Pittsburgh area.
As a bonus- there were 2 students filming a documentary on Halloween in the store during my visit- which only added to the festive atmosphere in the already crowded store.