I’m not here to question your intentions, but it depends on what you want.
You can pick up Trichocereus cacti (I think they are named Echinopsis now, but I’m not a botanist) at a local garden centre, they’re common enough an inquiry is not going to elicit any questionable stares or suspicion. Shipping should not pose any problems either but depending on your locality you might have to deal with phytosanitary certification or lose them to customs.
Trichocereus are a pain to keep and grow, Peruvian torch especially as they seem to have double the density of spines. I say this having grown many of them from seed.
Do you enjoy donning leather gloves and having to roll your plants in multiple sheets of newsprint for handling during transplants or grafts? Do you like manouvering top-heavy, spike laden, tipping-prone plants in the garden every time you want to water or observe your plants? Do you enjoy struggling with canopy and light heights to avoid etoliation and ensure even lighting in your garden area?
Are you looking for grafting stock? Buy a blue myrtle (Myrtillocactus Geometrizans). They are prettier, easier to graft, and have minimal spines. Mature specimens have only a small single black spine and can be comfortably handled without special precaution.
For those less interested in horticulture some vendors sell sacks of pre-ground cactus powder, undoubtedly chock full of ground up cactus spines along with anything else that might grace a clandestine enthogen processing facility. It would be more prudent to buy a blender on the way home from a garden centre.
As for vendors I cannot provide any specific recommendation. The prices I’ve seen from most cactus peddlers are enough to make me sick. These cacti are endangered and somebody wants to charge $300 for a 1" cactus, along with the exact geographic coordinates from which the wild specimen was poached? Speaking of course about peyote, as columnar cacti can be easily obtained from greenhouses which are not morally bankrupt.
There are legitimate greenhouses that do deal in seeds, but given the legality of peyote you need to look outside of North America. Pretty sure the dutch are cool with it.
Seems like just yesterday that I got started buying 10 seeds for $30 with a personal cheque through the mail.