In a tourism-heavy city like Barcelona, there are all sorts of specialized vendors of diverse sorts of souvenirs; I knew that but was still surprised to find a store that sells nothing but Rubik’s Cubes and their variants.
It’s tucked away in an off-street shopping bazaar, the Galeries Malda in the old city, a block or so off Las Ramblas. It was never busy when I passed it… and maybe that’s the real puzzle!