It's possible that they will never make more, but recently Square Enix released more Yuna necklaces in the wake of the HD remaster of X/X-2 (and previously, for the 25th anniversary). Don't give up hope; check the official store daily. Mind you, I would highly suggest checking both the NA Square Enix Online Store as well as the JP Square Enix E-Store. Yuna's necklace showed up in NA site, but some other items don't.
RIP CosRevo, Love Laugh, Sweety Pop, Cocosilver.