Japan prohibits quinine so the tonic water is not good (sweet like 7-up) if you buy one of the big names like Schweppes, which are produced in Japan.
I found a no name brand of tonic sold at Yamaya, which is a large wine & booze chain. It says "made in USA" on the can so I am pretty sure that it has quinine.
Donki used to sell Gilbey's gin and vodka for 700 yen a bottle. Maybe higher price now with Abenomics. I used to buy the vodka to make umeshu. I miss my umeshu. I tried to make it with various green plums in the US, but it does not have enough acid.
The beer to liquor price ratio is ridiculous in Japan. You can get a decent bottle of booze (or nihonshu for that matter) for the same price as 3-4 cans of beer.