Sure you can find knock off botox in Mexico. Any doctor doing it is USA is risking their medical license for life. Most legit dr`s with half a brain will not do that. Allergan is the only licensed provider for a reason. Their formula is secret but it is not just botulism toxin. Wanna see what happens if it was mixed at the wrong concentration look up botulism poisoning. Basically you will be paralyzed but can still hear, see, but not move. In extreme cases you will die from the toxin paralyzing you to the point of not being able to breath or have a heart beat.
There was a special on "American Greed" of a company in US selling knock off botox. They went to jail for a long time and over 100 doctors lost there license after a few people were hospitalized and almost died. Botulism was not hard to get for the people on that show when it was for "research purposes". Go ahead and get some.