If you want me to blow your mind right now, I will: You can buy San Pedro cuttings off amazon. I got two x one foot long cuttings off there for like 20 or 30 dollars. Confirmed. The guy had a ton and there were no bad ratings. I guess it's legal to sell it.
Both of the ones I bought took root. Idk if I bought from that seller, but they have good reviews it looks like, so take that how you will.
Update: The guy I bought from doesn't have listings anymore, so they may be taking them down. Regardless, the one I linked is still available. So...act now? I guess? I know when amazon gets a whiff of this kinda stuff they start to crack down (DXM powder, mimosa, dried B. caapi, etc) so it may pay to order before they do [note: I dont work for that seller or anything, just giving my opinion].
Note: I don't condone taking other peoples plants for any reasons, but you might be able to go ask the homeowners if you can take a cutting because you want to grow that kind, might go over well. Larger nurseries etc I find either won't know what you are talking about when you ask or will bullshit you and try to sell you the wrong thing.
That image is the cacti I received. Both have since grown six inches (in like 3 months) and taken root. So they are healthy.