It is called a Link Heart in Rejuvenation. I believe two of them can be found in Sheridian Wetlands in the Xen HQ, and after you complete the current version, you can buy them for 10k PokeYen from a Vendor on Route 3 during the day.
Try to use this thread next time if you need to ask a quick question about a item instead of making a whole thread over it -> Rejuvenation Ask Your Questions: Quests and Pokemon/Item Locations. Also, searching for the Link Heart helps as well.