Originally Posted by LapLap

If you're serious about the tonic water, you can get Fevertree in Japan (フィーバーツリー )
Hasegawa Liquors sell it in their website (190yen for 200ml - nearly same as in UK) and they have a store at Tokyo station.
http://bento.com/m/e/rest/3979.html

I'd suggest emailing them first to make sure they have it stock.

Alternatively, you could order from Rakuten and have them deliver a crate to your hotel at around the time you arrive.