Looks like the mystery has been solved!
Mika Noguchi posted about Ayu's outfit in the Days PV and it's made by Peach John!
Product page:
http://www.peachjohn.co.jp/shop/desc...7004e1f644bb7f
Blog entry:
http://www.1oven.com/mika_noguchi/ar...8/12/days.html
Peach John site about it:
http://www.peachjohn.co.jp/todayshot/081217/
It's the Peach John "long knit hoodie" and is available in offwhite, black, and pink for 8200yen.
Anyone who wants to buy it should get in contact with Celga or someone who can buy stuff from Japan.