In FFXIV, Isleberry Seeds are one of several varieties of items used to grow crops on an Island Sanctuary. They were first introduced in Patch 6.2.
Unlike more common varieties of Island Sanctuary seeds, such as parsnips, these are not found on the island itself and cannot be gathered infinitely. You must instead purchase Isleberry Seeds from the Produce Producer (X: 12.8, Y: 26.9). This is the NPC mammet standing at the foot of your crop plots.
One set of seeds costs two Islanders’ Cowries apiece. Each will then produce five Isleberries per seed you plant on your farmland after a default growing period of 48 real-time hours.
Other seeds purchased exclusively from the Produce Producer include:
Note that the Island Onion Set functions exactly the same as other seeds, despite the name.