Use Quinine Sulfate (not HCl). If you have a friendly doctor, have him write you a prescription for 325 mg capsules (not tablets). If not, you can order USP quinine sulfate on the web from a variety of places as a chemical. Aquarium stores often stock it as a fish medicine. Get USP in a sealed original manufacturer's bottle, and you'll be fine.
4 gallons tonic water (for a 5-gallon corny keg):
3 gallons plus 3 quarts water
1 gram quinine sulfate (or contents of 3 325 mg capsules, discard the gelatin capsule itself)
juice of 12 lemons (or 3 cups bottled lemon juice)
1.4 pounds sugar
Mix directly in the Cornelius keg:
Add lemon juice to water and stir
Add quinine and stir thoroughly
Add sugar and stir thoroughly
Close keg and refrigerate until chilled.
Carbonate to 60=90 psi:
Shake until you arms are numb.
Keep very cold, and back off the pressure when dispensing.
This stuff will not spoil, so you can also just keep it in a keg or carboy in the refrigerator, and fill a seltzer bottle with it and charge for a liter at a time.
Moderation in all things, especially in moderation