S.A.
I have bought costumes online for both myself and my son for the past several years. www.buycostumes.com has always beem favorite (they are located in New Berlin WI so even the (usually free) economy shipping is pretty fast)
halloween shops are springing up all over the place... I was in Arlington Hts over the weekend and saw a huge one at Golf & Arlington Hts Rds. A co worker of mine found a really cool handmade costume on ebay last year - the kind thats so well made the child can wear it repeatedly without it wearing out.