Dodgy bloke on the street, from a friend of a friend, Silk Road for hard-to-find things, some grow in fields, others you can buy the sources and extract them yourself.

friends, get friends

money too.

or just make your own

It's all about networking and exercising a little commonsense as far as safely buying drugs go.

Designer drugs are infinitely less painful to acquire from online vendors but you can still get burnt on occasion.