Lots of bars make their own tonic these days. So do a growing number of Chowhounds. One vital and sometimes hard-to-find ingredient for tonic is cinchona bark, sometimes available powdered. Recently, eyedocdiva revived a 2010 discussion with the news that Lucky Pork Market now carries cinchona. Just keep in mind, bigwheel042 warns, making tonic takes a certain commitment: "zesting all that fruit was a big pain."
Lucky Pork Market [Mission District]
2659 Mission Street, San Francisco