In Australia, you can buy a tonic mixer from Coles supermarkets and going by the faq on the website, you'll get more than enough Quinine after it's diluted.

Tonic mixer that contains Quinine $9

https://shop.coles.com.au/a/aspley/product/soda-press-co-syrups-3533971p

https://www.sodapressco.com/pages/faq

Does your Tonic contain quinine? −

Yes and it’s real quinine, not the chemically substituted version. It's sustainably sourced from the bark of the fever tree (Cinchona) and contains 0.76g of quinine per 100ml diluted. Natural quinine is a natural anti-viral, antiseptic and has anti-inflammatory properties. It is still widely used to treat and prevent malaria.

What's the right mixing amount? −

Ideally 1 part syrup, to 6 parts sparkling water. However, as peoples tastes preferences are different, it’s also good to Taste-to-Test. If you are wanting to make up a 1L bottle, fill the syrup to the start of the bottle neck after carbonating the water before-hand. Please note, the Kombucha Concentrates are 1 part concentrate, to 4 parts water.