For me in the USA, I definitely trust two retail supplement vendors - Life Extension and Vitacost(owned by Kroger). For brands, I trust Doctor's Best, Life Extension (they almost entirely sell their own brand), Jarrow, Thorne, and a few others. When we are in Thailand or Malaysia, we usually buy Blackmores brand of supplements which are sold at the pharmacy chains, Boots and Watsons. I am currently considering purchasing from RevGenetics.
For several years, I maintain an annual subscription to Consumerlabs.com, which tests many supplements for contamination, purity, and if the amount stated on the label is actually in the capsules or tablets, etc. They also consider the safety of a supplement and how well it works.
Dr. Sandra Kaufman wrote the book, The Kaufman Protocol, which discusses her recommendations for the best supplements for anti-aging. In her app and on her website, she has links to Amazon supplement listings. These are supplements that she trusts and I value her opinion. She participates in the forums here.