To start this off, you have to go to college and become a pharmaceutical salesman, once you have that under your belt, start up your own pharmacy, it being your own is very important. Once you own your own business, the connections will come to you for you to get the illegal drugs. After all of this, just ask the shady looking people walkig in your store if they want your drugs, and there you are, set for life.
(P.S. If you have any recommendations, tell me them in the comments)