Petunia Pickle Bottom
This store happens to have a very cool name, and lots of really cute products.
Petunia Pickle Bottom is an American store, but I found a website which sells their diaper bags (マザーズバッグ) in Japan too.
Here is the US site. Here is the Japan retailer that I used, and you can find some on Amazon Japan too.
My MIL told me to order / buy a diaper bag as a birthday present (from her to me), so I finally decided on this one (Boxy Backpack in Misty Shanghai) and ordered it online.
It was a little expensive, but has great reviews and some really cool features. Can`t wait for it to arrive.