Quinine is sold as an anti-malarial drug and is not available on the market in supplements because it is a controlled drug regulated by the FDA.
There is one homeopathic formula product on the market that contains Quinine Sulfate, but because it is a homeopathic remedy, it essentially contains no measurable dosage of Quinine.
The only way to get Quinine outside of a prescription drug is by buying tonic water (when Quinine is listed as an ingredient on the bottle).
“The foundation of traditional tonic water, quinine, is a bitter tasting alkaloid that is derived from the bark of several species of the genus Cinchona.” [source]
The homepathic remedy Chinium Sulphuricum is the homeopathic version of Quinine.
“Quinine is an alkaloid synthesized from the bark of cinchona, a tree native to South America.” [source]
Learn more about the background and history of Quinine at the links below: