Dana's dress and other non-player character outfits from Adventure Academy are not available for players to purchase for their Avatar.
However, there is a similar dress to Dana's that is available in three colors. It is called the Polka Dot Dress and can be found at The Clothespin. 
Please note that like all shops in Adventure Academy, the items displayed at The Clothespin update every 24 hours. So if you don't see the Polka Dot Dress when you check at first, don't worry! Check back in at The Clothespin frequently after its inventory refreshes, and the Polka Dot Dress should eventually be available to purchase.
The Clothespin shop is located in the Marketplace. For directions on where to find this shop, please see the following article: