The Oceanic Insitutute in Hawaii is doing some great work with aquaculturing yellow tangs and you can buy them through Biota. They're ramping up production so hopefully the supply will be more consistent.
I grew up in Hawaii and have seen how the reef has deteriorated dramatically between the 1990 through 2017, when I left. Between silty, nitrogenous runoff from overdeveloped land, tourists trampling on living coral, overfishing, oil spills, sewage spills, etc. the reef has taken a beating and it's only getting worse as Hawaii gets more (over)developed. I couldn't find any studies about the current state of yellow tang populations, but a 2020 study of West Hawaii waters (where a lot of fish for the pet trade are collected) showed a 45% decrease of fish biomass over just 10 years, 2008-2018, among grazers like tangs.
So I get why the folks in Hawaii want to protect their waters. What I don't get is the "right to fish them for food" thing. I know people eat parrotfishes (uhu) and goatfishes (kumu) but never heard of anyone eating a tang and you kinda don't want to eat reef fish anyways because of ciguatera poisoning. But I'm not Native Hawaiian so what do I know?