ebay. For the past few years, I've purchased my kids' costumes through ebay and found some great deals. They have nearly every costume ever created and with a variety of sellers and options (new or used, bid or buy it now) you can find the deal that is right for you.
These baby animal costumes are all brand new and all available on ebay for $18 each with free shipping! I've seen the same exact costumes sold through other retailers for double the price. So if you are still looking for the perfect costume, try ebay, friend!