Tonic water and other mixers in lounge/café cars are provided for the employee to use as mixers for alcohol that is purchased in that lounge/café. There is no pricing structure that allows for separate sale, and the limited supply means this would cut into the supplies needed to provide for paying passengers. Alcoholic beverages that you bring on the train may not be consumed it in any public area. A private sleeping car room is the only place where it is permitted .
Assuming you're in a sleeping car, just bring your own, I bring either a six pack of small bottles or two half liter bottles, along with my gin and limes, ice is available in the sleeper. You can buy vodka tonic/gin tonics in the lounge, and usually in the dining car too. Now if they only had Bloody Mary mix for breakfast!