Your statement contains the idea that LSD is the pure, platonic psychedelic. In fact, LSD is just as synthetic as the other chemicals you are discounting. It is actually named LSD-25, as Hoffmann took the base molecule from the ergot fungus and methodically tweaked bonding cites around the rings, and LSD-25 just happened to the 25th variant he made. Another one he synthesized is commonly known as ALD-52; it too is available in online markets.

For sure there are dangerous chemicals that are available too, but there are cheap testing kits to let you know if you are dealing with a true lysergamide or one of these more dangerous RCs. If I was to buy LSD, I'd feel far more confident buying it from a reputable online vendor than some dude in the parking lot of a Phish concert.

As for mushrooms, the amount of psilocybin in a gram of mushroom can vary by a factor of 4, so even though it is a natural source, it is still a crap shoot.