Go to your nearest fabric store/hobby lobby/walmart that sells fabric, and flip through the pattern books. Most of what you find won't exactly be helpful, but you are likely to find a starting point there, also make sure to check the Halloween costume book, generally it has more appropriate patterns for what us cosplayers need.
Just try to find a pattern with the same basic shape or general style of what you need, or you may need to mesh two patterns together to get what you want(for example the skirt of one dress with the sleeves of another), but with a bit of creativity you can do it!
Also check out http://www.simplicity.com
They are the two brands that most stores carry.