I'm going to assume this is about Altis since it is in the Altis sub forum.
Go to the Cocaine Field marked on your map. Once there you will see a wooden pit with green leaves inside. Simply go inside it (press v to vault over the edges) and run around in there to 'stomp' on the leaves. You will get unprocessed cocaine in your inventory, however this is useless unless you want to craft weapons with it. If you want to make money, you have to go to cocaine processing and process it using the npc before selling it at the drug dealer.
You're lucky, won't find any dispensaries in the UK because the government is shit.
Although it's not all that bad, my guy had sour diesel last time, fuck knows how he managed to get that smuggled in/imported.