Look for agroveterinarias. It's a place they sell veterinary and agriculture products. You can even find pictures and specific examples of agroveterinarias on the Peaceful pill handbook. The brand name is different in each county. For example. It's called Penta hypnol in Peru, Euthanyle in Argentina, Dolethal in Mexico....
If you ask for pentobarbital they might not know what you are asking for. You have better chances of finding N in smaller cities.