I simply plop down a Sim on the Gallery who already wears the beret, then you can make your sim be able to wear the beret too since all the unlocked clothing items from that Gallery Sim have been transferred to the household of your choice. Chances are, they'll also have other unlocked CAS items available to wear as well, like career outfits/attire, skill-locked attire, and retail/restaurant/vet/club attire! No mess, no fuss, and no need for mods!
But yes, it's from the Critic career from City Living. I believe any of the branches will net you the beret, though I propose trying the Art Critic branch first if you instead want to work for it.