Thanks to VOVO for finding and mentioning our site. I am JD, the founder of onlybitters.com and onlytonics.com based here in Melbourne. We import and carry 12 varieties of artisan tonic syrups with two more on their way. For all your home tonic syrup makers, you should look into using some of these as a reference point for what is commercially made by small batch suppliers (nearly all from the US and one from Germany). The syrups range from the very bitter to the very sweet and everything on the spectrum and in between. One supplier (5by5) interestingly is not using cinchona bark but gentian root to bitter their tonic (to great debate in US circles if it is still a tonic syrup), but customers who have bought it, love it - so who am I to argue.
On the topic of cinchona bark, I have started discussions with AQIS to see of I can import the bark itself. I will also be bringing in the major extract used in making both bitters and tonic syrups. So keep an eye out for that. I will update the forum once I have an update on this. In the meantime, enjoy your G&T's!