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.