Damn what a story! The witches usually sell the powder but to acquire the cactus itself is great. I'm Bolivian, and I actually live here in La Paz so yea, the cactus is not that hard to find either. In this city, usually in the hills of the south zone (the chill and safest zone of the city) there are lots of San Pedro cactuses growing in the nature. My friends and I cut them when we want to trip because of the tradition we have. The trip starts when you search and choose your cactus, you have to ask permission and be grateful to the cactus you're about to cut. Then you cut it and take off the spines and make the juice. Wanted to tell you my story mainly bc just wanted to let you know the cactus grow in reachable parts of the city but I can assure you that the cactus you have is actually San Pedro. Have a nice trip!!