I made a table where you can find out the source/location of factory for where health supplements are made. This is a work in progress but it functions. At the moment you can sort by cGMP/GMP certification.
* Full disclosure - if you purchase anything using the links, i earn an small affiliate fee (5%)
I made this to help demystify the language behind product labelling, which in many cases is obfuscating the manufacturing and sourcing of certain products
Current Good Manufacturing Practice is a good thing but in the supplement industry is mostly an attestation. Unlike drugs, supplement manufacturers are usually not evaluated by the FDA unless they have collected evidence of problems at the facility. It requires people designated as quality control officers, written practices, and storage of samples, but, unlike drugs, nobody checks this stuff before a supplement is sold. The supplement industry has had some powerful congressional allies, and it is possible that too much regulation would cause congress to revoke the FDA's authority over supplements entirely. (My speculation).
Another standard is USP. This standard includes GMP and goes beyond it, with verification of potency, vehicle breakdown/release of ingredient, and measurement of contaminants.
There are several well-known brands that are consistently on-target with their supplements, and others that are frequently off target. I haven't reviewed the data recently so I won't list them here.
https://www.consumerlab.com/ https://www.usp.org/dietary-supplements-herbal-medicines https://www.quality-supplements.org/