Easley's Fun Shop is a fantastic place to find nearly any costume you can imagine. You can rent or buy most costumes or you can piece them together on your own. Aisles of fake moustaches, gaudy jewelry and other costume props ensure you will find what you are looking for. If you need something a little more along the "prosumer" line (wigs and mustaches for professional theatrical productions) there is someone on hand who can assist you with what you are looking for.
Don't be surprised if you discover a costume you hadn't imagined existed. There are dressing rooms for you to try them all on before you purchase them, as all sales are final.
Party supplies are also plentiful if you are throwing a theme party for a friend who might be going away or turning 40. Scads of silly (and maybe a little naughty) party favors will make your gathering one to remember.
If you are going in around the end of October, keep in mind that you are going to be faced with a madhouse. When I went about 2 weeks before Halloween, it wasn't so bad but business was still fairly brisk. A few days before Halloween, there was a police officer directing traffic in and out of the tiny parking lot. If you plan ahead about a month or more you should have no problem finding exactly what you are looking for.