Can't you just buy alcohol from a vending machine?
(Source: Bought a lot of alcohol from vending machines while I was in Japan. Never got asked for ID.)
(Source: Bought a lot of alcohol from vending machines while I was in Japan. Never got asked for ID.)
EDIT: Found a source "In May 1995, to prevent underage drinking, the All Japan Liquor Merchants Association decided to eliminate outdoor alcohol vending machines, so it is now rare to encounter these machines outside." https://www.nippon.com/en/features/h00258/vending-machines-i...