Try hanging out in pubs and places like that, and LISTEN to people - people that talk about cannabis (you'll be surprised how many do - it's illegal to smoke, not to talk), approach them and start a conversation. Then ask if they wanna go outside for a smoke. If they like it, ask if they want to buy some. Give number. Simple - you've now made a customer and a friend.
One thing though - don't deal to chavs. They'll only fuck you about. Deal to the decent friendly people.