'A' seems definitely safer than trying to buy N by picking out a random vendor on the darknet. as far as i know china had it banned, so if someone is offering you "fresh N powder imported from asia" better be careful. i also wouldn't buy N as powder form since it looses its effects faster than the liquid in the sealed bottle. one hole in the bag and it may arrive at your place already effectless when it was stored a long time.
same goes for drugs from darknet, i only know about the european shop "shiny flakes" some people around my town ordered drugs from that page. but it's gone
i'm wondering if anyone ever thought about synthesizing barbiturate analogues that surpass the drug law like other research chemicals, there's also fentanyl as rc variant. but i don't know about its potency or any trusted sources, i got very inactive with that stuff :( i can only give you a tip where to order ketamine but that's not really useful for ctb, one can combine it with other drugs since it has breath depressing effects too.