Konnichiwa! Welcome to Tokyo!
Take a look at my published guide on playing Pokemon Go in Tokyo: https://pokemongohub.net/user-guides/tourists-guide-pokemon-go-tokyo/
While I was in Japan, I happened to find Farfetch’d in local parks but also at major modern tourist sites.
If you’re looking for a large park, I recommend Ueno Park. It is easily accessible from JR Ueno Station.
If you’re looking for a largely developed tourist area, I suggest heading to the Odaiba district. It’s full of Pokestops and it’s also surrounded by water, which is where you’re more likely to find Farfetch’d (at least from what I found).
Hope this helps!