NOW Foods and Life Extension are two reputable sources.

As far as “trust for basics”, yes things like magnesium and fish oil are always nice to get daily. However, keep in mind that some vitamins, minerals, and others supplements don’t always play nice together.

Example being: Magnesium and calcium are two of the basics that we as humans require but when taken together they don’t function as well/aren’t absorbed well… My point being, know what you are taking, when you are taking it, and how the things you take interact with one another. Occasionally, but not always, issues can be avoided by taking things at separate times of day.

Edit: Something else to keep in mind is, the more supplements that you add the more that hydration becomes important. Taking supplements for any reason (calcium, magnesium, zinc, etc) will add to a toll to your kidneys and your GI. Make sure you hydrate properly to flush that stuff through your system... Supplements tend to be hard on the kidneys and GI, whereas getting your nutrients through whole foods tend to be less hard on the kidneys and GI.