I'd be very skeptical of anybody advertising a substance on a public forum, particularly RCs.
I can say I've seen SR-17018 listed a few places I know to be reliable so its out there, at least to some degree. I will not share sources for obvious reasons but some advice for buying it:
Always buy small at first, so if you get scammed you won't be totally screwed over.
Ideally get it from a market instead of just some guy with a menu.
You'll want to figure out cryptocurrency, don't just give somebody your card or bank info.