Tonic water is a drink mixer made from quinine which is the bitter essence from the cinchoma tree. Tonic water is sweetened and flavored with fruit extract.
Tonic water is a standard beverage product available in the soft drink or liquor section of most grocery stores. You can also purchase this online at stores like Bevmo.com. A good tried and true brand is Schweppes but some new comers to the market like Fever Tree (shown) produces a very good quality tonic water.