You basically can't find the same dresses in two different places. Each place that sells dresses has a slightly different dress.
The ones in the parks are the most expensive. As others have said, $65 and up.
The disney stores in the mall will carry some styles at various times of the year. They are also quite pricey but sometimes you can catch a sale.
You check the
Disney Store online and see what they have. Not sure of the stock at this moment since it's not Halloween time.
All of those dresses come in sizes, like small, med, large.
We have been able to get several princess dresses at
Walmart or Target. They are one size fits all, so you have to take that into consideration, but at $25 the price is hard to beat. The styles on these dresses change frequently as well, and the 2 different stores may carry different styles.
We have a "we don't buy costumes at
disneyland" policy because I just refuse to spend $70 or more on a costume when I know I can get them other places for less. And even the "sized" dresses don't fit DD correctly. I usually have to shorten them.