They are sold both on the clearnet and the darkweb; however, many sites selling them are outright scams (if a site on the clearnet claims to be selling substances like a-PVP, methylone or MDPV, which have been explicitly scheduled for years, you can be certain it is scammer; even the semi-"legit" ones, however, will often substitute a popular cathinone for an inferior analogue).
Hodor's answer is as good as you're going to get I'm afraid, as this is very close to asking for sources. Therefore I'm going to have to close this thread. But good luck in your quest... :D