It'll be hard to find a dress that's just like Ayu's of course. Luckily, it's a plain dress, so most short, full-skirt, white dresses could work.
I recommend these (which can be ordered in white), which are all under $50.
http://www.simplydresses.com/shop/viewitem-PD525792
http://www.simplydresses.com/shop/viewitem-PD531146
http://www.simplydresses.com/shop/viewitem-PD443059
BUT MOST OF ALL, check out this one. I think this is the closest you'll get to the original.
http://www.simplydresses.com/shop/viewitem-PD536762