You usually have to buy them, either off the internet or in fabric or craft stores, but there are tutorials for making patterns online which are more likely to be free. Usually, all you need is large sheets of paper, a measuring tape, and a lot of time and patience.
http://funadvice.com/r/bg309ivjnge There are tons of them. If you want a specific kind of dress, I'd search for that, along with those criteria. They usually make it pretty easy for beginners.
you know some online pattern making?