Independent costumes stores around MinneapolisTwin Cities Magic and Costume in St. Paul have thousands of costumes and their staff are very helpful if you are stumped - they'll help you with the outfit, acessories, wigs, shoes, masks, makeup and props you'll need. You can also hire professionally made costumes from their stock.
And if you have the DIY spirit, the four Ax Man stores in St. Paul, Fridley, St. Louis Park and Crystal have plenty of supplies. If you'd like to do a mad scientist, they have lab coats and laboratory equipment. If you'd like to do something military-inspired, there is army surplus clothing, boots, gas masks, and vintage army medical supplies. Steampunks and Indiana Joneses will enjoy the bins of leather scraps, chains, springs, gears and random mechanical devices. I recently picked up a selection of fake birds to do a Tippi Hedren costume from the eponymous movie.
Vintage stores in and around Minneapolis are a perennial favorite for costumes. Be a flapper, swing dancer, fifties housewife, disco dancer, teddy boy or hippie. At this time of year, vintage stores often bring out their less than perfect garments, and the truly hideous relics of past decades, and sell them for costumes.