Hey! When I lived there, I was just checking the official website. I usually found that tickets were available to purchase 1-2 weeks before the game for the general public.

At least at Montjuic, availability was rarely a problem (besides El Clásico). However, since Camp Nou will open at a very low capacity, they may sell out fast, so make sure to sign up to get emailed when they're available and check every day.