Fly agaric (Amanita Muscaria) is indeed a deadly toxin that can be fatal with an overdose (easy to do).

All shrooms are usually "homegrown", there are no such things as legal labs to grow them in Just get them as fresh as possible and from reputable and trusted dealers/growers. The psilocybe cubensis strains are the most common and will all be around the same potency (dependant on the grower and the substrate used).