You can find liquid Kava through Herbalist and Alchemist in the US. I don't think they sell retail, but you could stock it and dispense it.
Kava is quite strongly bitter. I would dose 0.2 ml per 5 lbs dd BID, or more often prn (e.g. before and during thunderstorms, etc.).
Milk thistle doses are highly variable, partly because of product variability, and partly because it is such a safe herb. Ours is about twice as strong (concentrated) as retail brands. For ours, I generally use a dose of 0.2 ml per 5lbs BID.
Good luck,
Steve