The guy standing behind the bench I was sitting on behind some artists on El Caminito today was puffing away. Maybe he could help you.
I also smell it around the Facultads in my barrio... so maybe you just follow your nose.
If the weather is nice tomorrow and/or Saturday, you should be able to smell it in and around Parque Las Heras. People will be tanning and toking and you could maybe hit them up.
suerte con tu mission