You should be able to find smoke on the beach or on the street corners at night. Be careful not to get ripped off. The cops usually harass people on the off streets. They can get bribes easier that way.
Learn: Donde puedo encontrar buena mota (marijuana)?
Where can i find good marijuana

Donde puedo comprar mota (marijuana)?
Where can I buy some marijuana

Bring some papers unless you don't mind gutting a cigar.