A few months ago, I was looking for sodium pentobarbital on the dark web. I found only two users who were "selling it" both on two different markets, while one of those markets was relatively new.
The problem was, unlike other substances that were on offer there, these two users had no single review written about their pentobarbital sales, they did not even have a single successful sale of this specific item shown in their sale history, one of them called it nembutal, and even though they priced it for, I think, 600 dollars per 10 grams, the biggest red flag was that they both claimed to be selling it in powdered form, with pictures of white powder in a plastic bag attached to their posts.
And as for the veterinary, liquid version, I did not manage to find a single bottle on offer anywhere while searching some of those markets.
So, I am not sure I would really trust dark net markets with this right now. Or at least not until there is a potential veterinary version on offer.