Replacement stickers
Since the set is still being produced, you can call Lego Customer Service at 1-800-835-4386, and they will most likely be able to help you.

Failing that, the BrickLink website is a popular place to buy replacement parts. Here's the link for the sticker sheet:
http://www.bricklink.com/search.asp?itemID=102237

It looks like there are a handful of sellers in the US selling the stickers for $1-$2. Registration on the site is easy, and most of the sellers accept PayPal.