It's not even on the darknet. Honestly, I'd give up on this elusive substance. There's no point in chasing it. The only practical way of getting it is robbing a vet. I wouldn't be inclined to do that though, and I wouldn't even entertain the idea. But the only people with access to the substance are vets. You'd have to find a corrupt one and that is nigh on impossible.
If you were one of the fortunate ones who got it from China when it was doing the rounds more then 10 years ago, then yes, you would possibly have it...but these days it's not going to happen. The world is getting harder to live in, it's getting impossible to afford to even live, and yet here we are with drugs or other substances being impossible to get hold of.
There is no easy way out of this. The world was tough about 90 years ago, but you still had access to barbiturates. Soon, the world is going to get as tough or maybe even tougher, and you won't have access to anything.