If you don't get any hits, try Payless Shoes. Many have a women's size 11, certainly online up to 12 (but first call the ones you plan to visit).

You can see first whether they would have anything you'd like:

http://www.payless.com

Click "view all" then sort by color. You could then even call one of the local stores and say "do you have Natalie Bow Pump in white size 11?"