@whitefeather are you around.. this is an exit member I saw them post ..?
It's basically all over south America. They list two main countries but that's because some people had luck there. I think the best bet is to go to a country you have friends or family, or a country in central or South America that you would like to visit and check the pet stores or vets. If everyone goes to the same place they might get suspicious. I think Colombia has it but is not mentioned as much. Brazil might be a good one too. I would think the poorer areas your money would go further. Out of the way places or industrial areas it might be over the counter, or at least easier to obtain if you have a $100 bill or two.
I would think you might find it if you hit up a bunch of pet stores/vets. If not you can get a fixer, ask a cab driver or something for the inside scoop. I think if everyone rushes to the few cities in two countries mentioned in the handbook we limit ourselves. Only vet N is available, You might want to give a story about a horse you need to put down. Maybe even have a picture. I suppose a Spanish speaker might help, or a local if you could find one. Maybe the cab driver can get it? This is partially out of the handbook, part speculation. You should buy two bottles if you can.
I have a hard time traveling. I am probably going to send someone. If they ever get it, I don't know if I will even try to bring it back. I might fly there and use it.