The worse-looking (grotty, run-down, probably mostly sells cigarettes) type of souvenir shop has the best chance of selling postcards, in my experience. The photographs might have been taken in 1995.
I've been to some countries where only the official post office was allowed to sell stamps. That makes things much more inconvenient than the shop selling postcards also selling the stamps.
I mean both that they are used much less and they're much less available.
You can find postcards of the Coliseum in Rome, but you'll have a hard time finding one of Palazzo Farnese in Caprarola.
Even in major touristic hotspots, 30 years ago you'd find postcards sold at any shop, but these days you have to look for a while, in my experience.