Anything specific would be inappropriate for this forum. But the broad answer is a chemicals distributor in China. I actually only utilize them for legal "nutritional" compounds, but others use them for pharmaceutical options, and those are independently tested frequently by a variety of stakeholders. Personally I really wish US citizens didn't feel forced to go outside of the quality protections of the FDA for their medical care.
Nutritional compounds? My understanding is that without a lot of special processing, GLP-1 agonists (semiglutide, liraglutide, tirzepatide, etc) will be degraded by stomach acids into useless fragments. There's an oral semiglutide formulation but it looks like it's quite inefficient and has some pretty strigent requirements in order to be effective.
What? No one is taking these orally. Hence the bacteriostatic water for injection.
The same distributor sells legal things to import like L-Carnitine, Epitalon, Berberine, NAD+, NACET, and vitamins at great prices so I was saying I use them for things like that.
But others use them for GLP-1 agonists and many do their own independent testing.
