Tonic water as we know it in the west is available in Japan, but can be difficult to find.
You can buy imported from the U. S. A. in 355ml cans or Feavertree in 200ml glass bottles at Yamaya stores. Just Goggle *yamaya near me" to find your nearest.
There usually in the larger city's & some have multiple branches. There's 3 in Osaka for example.
You can also find a Japanese brand "Wilkinson" and whilst this doesn't have quinine in it, it does have an alternative bitter flavouring. So it's drinkable.
The tonic water that still eludes me is a sugar free version, but I'll keep looking!
Generally any sugar free drink is difficult to find in Japan, the Japanese obviously aren't bothered about consumption of sugar as some of us westerners are!
I have found one alcoholic drink that is sugar free and is sold just about everywhere.
"Kirin strong zero" it a vodka base mixed with various fruit, orange, lemon, grapefruit etc. It is available in different strengths from 5% to 9% ABV, so be careful with the 9% stuff, it's very cheap as well a 550ml can of the 9% version can be bought for as little as ¥150 (£1.06 $1.37 €1.26)