You've got to get these nodes in order to be able to buy the truffles, connecting them in this order:

Velia > Luivano Island > Paratama Island > Beiruwa Island

After you do that, take your boat/ship and go to Beiruwa Island, buy the truffler from the node manager and put them into the ship storage, sail to Altinova's docks, put the truffles in your inventory and sell them to Nyabi (and don't forget to talk to Tunger at the docks before doing so)