The highest rated online store with the most costumes available is Buycostumes.com. Buycostumes has over 10,000 costumes in stock that come in all shapes and sizes.
At Buycostumes.com, the prices range from under $5 to over $100, as of July 2015. They offer you the option to select how much you want it to cost, for what holiday the costume is intended, what type of costume you're looking for and many other options. They often have special offers and their costumes are shipped internationally.
Buycostumes is just one of the many stores that offer costumes for adults. Other websites like CostumeSupercenter.com and HalloweenCostumes.com have also been recommended by many a party goer and are also websites that offer all kinds of costumes with quick shipping.
Buying costumes is only an option. There are several simple costume ideas that can easily be crafted at home by using whatever you have stored away in your closet, or with cheap materials available in any nearby craft store or thrift store. These can be simple things, like putting on an old black and white striped shirt, some black trousers, a black beanie and a black mask - or even a painted-on mask - and you could dress up as a masked bandit.