@UKscotty people are referring to Pentobarbital and that "Nembutal" is just a nickname.
People should learn not to conflate those two or use them interchangeably, because it is wrong to do so.
Nembutal was a brand and a medicine.
Pentobarbital was the main substance in Nembutal.
Nembutal doesn't exist anymore because it has stopped being used in human medicine.
Pentobarbital It's still widely used in veterinary medicine, under various brands, where is the main substance.
Those veterinary brands are what people still buy from South America.
The only exception in human medicine is the availability of raw pentobarbital (powder) for assisted suicide. But the sale of this is only reserved for a limited, and particular, number of clients. It's not available on a large scale.
I think that the biggest source of misinformation regarding this is the Peaceful Pill Handbook. It contains a lot of outdated and even wrong information, and not only regarding this substance.
When reading PPeH, people should always corroborate the information from there with other medical scientific studies and not take it at face value.