Ask the street dealers or alternatively, you can buy easily drugs on the dark web. However it's a bit more expensive option and slightly more tricky. Just download Tor, make a connection, use a search engine or hidden wiki.
However firstly, read very carefully tips how to browse through dark web safely.
It's very important to gather as many information about safety as you can because you can't browse the darknet as you would browse the "normal" internet. If you are not interested in technology then arm yourself with patience - it's not gonna be the most interesting read. However it's very, very important to know how to use dark web safely. Otherwise you can get hacked or scammed.
You should spend some time to figure out which sites are scam and which ones are legit. It is a tricky part as even if a site is generally legit, the seller might not be. However on most legit sites are systems and technologies which secures the buyer from fraudulent sellers. Then just make an order and here you go. Keep in mind that fent is generally not very cheap and on the darkweb you will pay slightly more than on the street.