Hello! Drugs that are legal with a prescription in the real world typically still require a prescription in Duplicity and wouldn't be available over the counter. Recreational drugs, both the real kind and various made-up drugs which only exist in Duplicity, may be sold in some specialty shops or on the streets via dealers depending on what they are. They're not all legal in the regulated sense; it just isn't illegal to buy, sell, use, or possess them.

Some black market vendors would sell directly to Submissives, while others would not. You are correct that there is always some level of risk!