is there really enough quinine in tonic water to make a difference?

This is a good question that we lack sufficient evidence to draw firm conclusions on. One of the things that I've read to "debunk" the use of Quinine in tonic water is that to get the equivalent of 200mg of HCQ you'd have to drink a couple liters of the tonic water. But this presupposes that you actually require 200mg for effect against coronaviruses. Which we lack evidence of. What we do have is centuries of anecdotal evidence that quinoline compounds and their various analogs (berberin etc) are effective against viral infection. Elderberry has been used for a long, long time against influenza.

Ivermectin I can get from vet supplies stores without a prescription. I guess it will depend on your jurisdiction and local laws.