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It's a bit of a legal grey area here, there are members only coffee shops, or asociaciones where you can sign up and get some amazing quality bud....
Don't waste your time buying on the street, totally unnecessary in Barcelona since they have almost legal associations, like members only coffee...
There are now lots of associations in Barcelona that are "legal", the best I've found is called Zion, Calle Ginebra 31. Nice people, lovely vibes...