BuyCostumes.com is a major US based costume retailer headquartered in New Berlin and Wisconsin. It is an e commerce company that sells costumes and costume accessories for children adults and pets as well as party supplies for year-round occasions and seasonal indoor and outdoor decor. It is part of a family of costume and party brands.BuyCostumes.com is a major US based costume retailer headquartered in New Berlin and Wisconsin. It is an e commerce company that sells costumes and costume accessories for children adults and pets as well as party supplies for year-round occasions and seasonal indoor and outdoor decor. It is part of a family of costume and party brands.