Disney costumes are very popular, you may know someone who has one lying around that would fit your niece and not have to spend a dime on a one time use item. If you wish to buy it try http://www.buydisneycostumes.com/
Most major costume stores sell Disney Princess costumes for children and adults. You can also find costumes at the Disney Store or at Disney theme parks.
Shrek costumes are approved by Disney, and are licensed.
The best selection of Disney costumes is at disneystore.com. However there are many retailers that sell various Disney costumes. If you prefer to shop in store you can always find Disney costumes at Wal-Mart, Target, or Sears. If you want to shop online you can find Disney costumes at buycostumes.com or spirithalloween.com.
All costumes worn at Walt Disney World and the other Disney theme parks are designed by Walt Disney Creative Entertainment.
I think there can be Disney costumes at a Disney store at your local mall, you can go on amazon.com/ca (depending on where you live) Amazon is very reliable.
You can buy costumes in Jester's Costumes in uptown .
Disney character costumes can be purchased through online retailers like Disney and Amazon, or in a local Disney store. Occasionally, they may also be found in local classified ads and on auction sites such as eBay.
There are many different places to purchase costumes for kids such as the Disney Store and also on there website. Around Halloween places like Walmart, Costco or your local costume shop will have lots of costumes for sale.
The Disney store would have great, high quality, officially license Disney costumes for any and every Disney characte ryou can think of. If you want more affordable options, try Walmart, Kmart, or Target. Partycity may also have some available.
Cute tween costumes could be a cupcake, a pirate, or a Disney princess.
You can buy Final Fantasy Costumes at the related link below.
Yes, many of Tinkerbells friends have costumes. These can be found any place selling costumes around Halloween and at the Disney store.