IF you come from overseas to Tokyo (or from Tokyo to overseas) by air, your landing airport would be Narita Airport or Haneda Airport. Especially, Narita Airport handles more than 90 airlines, so, most of you arrives at this airport.
The airport is composed by the terminal1, 2, and 3. Terminal 3 is only for LCCs.
Terminal 3 is smaller than the other terminals. If you wanna enjoy shopping at Narita Airport, I would suggest to go to Terminal 1 or 2.
You can buy a lot of souvenir at the shops in Narita Airport.
Or you can go to an outlet mall by a bus (paid service).
(Reference: Shisui Premiam Outlet Mall Website )
FYI: According to Narita Airport Website, the airport has begun since 1978 and the number of passengers is going to reach to 1 billion in 2017 summer.
They are holding the anniversary SNS contest now. (Japanese Only: https://narita.campaign.voyage/1billionnarita/ *The deadline is Aug 31st 2017.)