You want to buy from a physical shop? A discounter like Kakuyasu might be your best bet. They also deliver, which might come in handy unless you're planning to chug the whole case right there in front of the store:
https://www.kakuyasu.co.jp/store/com...0010/00071018/

Store list:
https://www.kakuyasu.co.jp/store/app/shop/shop_list/