The spores of mushrooms containing psilocybin are not illegal, so you can buy them and grow them yourself. Takes about 3 months.
DMT can also be homemade, because it's found in like every plant in trace quantities. Have to order a bunch of plant matter and cook it down.
Cacti containing mescaline, e.g. San Pedro are also legal to sell, can be cooked down to extract.
Dextromethorphan, the active ingredient in cough syrups, is a potent dissociative, but I would strongly strongly recommend never doing this drug. It is demonic and tripped me up more than any other in my life
And of course there are "research chemicals" that can be obtained which include analogs of LSD and DMT as well as bunch of other weird shit. These can actually be tricky to obtain because the companies selling these want to avoid the legal headache if someone ODs, and they're also probably the most unsafe because very little is known about them. I would prob avoid these entirely.