After adding the 4dkh water to your drop checker, which pH regent do you use? I have an api master freshwater kit and I'm not sure which one to use, either the high range regent or the low range regent?
Use the low-range, from my experience I feel 3 drops to be enough for a nice royal blue color.
As for a source of 4kdh, I just purchase mine from greenleafaquarium.com when I'm purchasing other supplies.