Easy Drug Rehabilitation

By this I mean, what sort of drugs which are illegal at street level, could be found in professional establishments such as a pharmacy or a clinic?

This is because of some research I am trying to do about drugs for my Personal Social Development class.
Also can I have specific examples, with the drug name, and the sort of establishment it would be found in.

I came across some websites on the internet wher you can buy prescription drugs without a prescription. Is this legal? Has the FDA even said it is safe to buy prescription drugs online without a prescription from a medical doctor? Personally, I think the government should shut down online pharmacies that are selling prescription drugs without the necessity of having a prescription.