In addition to the legality of purchasing a product like Botox online (it's not legal to get it anywhere other than from a physician's office), it could be a very big health risk to do so.
Botulinum toxin is a very dangerous compound when not properly controlled. Anyone selling "knockoff" Botox online is likely to not have the kind of manufacturing, shipping and quality controls in place to properly make, store and ship this kind of product. That's the reason why the real thing is so much more expensive, because the way it's manufactured, handled and stored is extremely important to the quality. In the best case, the product might not work for you. In the worst case, it might kill you.
Some of our competitors purchase Botox from overseas and we honestly pray for their patients that they do not get a "bad" batch.