I was raised to scorn store-bought costumes. A few snuck in, but in general, we got more attention for being weird or easy than for having the right characters. My brother was an army guy several years running (we had camouflage shirts and jackets and lengths of bullet belt thingies around, easy), my sister was a table one year, I was a traffic light, a bird in a birdcage-- oh, the pirate, that was my brother, with a Certified Live Parrot (taxidermy from Dad's bio classroom) on his shoulder.

Besides, store costumes are freezing. All good costumes begin with a sweatshirt.