My favorite supplies is NootropicsDepot.com. I trust them to be high quality, and their product pages have long descriptions of what the thing is and why they believe you might want to take it....
Supplements are a mess. I do a full blood panel once a year or so and end up supplementing D3 and B12 a lot, but other shit I just ignore pretty much because even if there’s an effect I expect it to be very marginal, ie any benefit being noisy and small enough that it’s not worth...
I take fish oil, mostly because I’ve come across multiple large studies over longish periods of time talking about it and what looks like big effect sizes. I should probably get a blood panel more frequently, and use that to steer some other bits and pieces....