IMO /r/microbiome is a great resource. Reddit in general, eg the nootropics subreddit is a treasure trove also. You could also search for „ameliorates [insert problem] pubmed“ or „[problem] [potential solution] pubmed“ in the search engine of your choice, and if an abstract piques your interest, view the full paper on sci-hub.
One thing I‘d like to stress is that the internet is full of scams and counterfeit products, and you can only be (pretty) sure a supplement is legit if you order it directly from the manufacturer. I think iHerb also has really good quality control, FWIW. Patented extracts are IME also preferable over run of the mill extracts, some of them are orders of magnitude more effective.
One thing I‘d like to stress is that the internet is full of scams and counterfeit products, and you can only be (pretty) sure a supplement is legit if you order it directly from the manufacturer. I think iHerb also has really good quality control, FWIW. Patented extracts are IME also preferable over run of the mill extracts, some of them are orders of magnitude more effective.