Large Fishing Net is a consumable on FarmVille which is used for opening Large and Extra Large Sea Treasures on water part of Hawaiian Paradise (farm).
It can be obtained from asking friends, free gifts or as reward for finishing some quests. It can be bought from the Market at a cost of 
1 cash for single Large Fishing Net. There is also other packs with more this consumables - pack of 10 Large Fishing Net cost 10 cash and pack of 25 Large Fishing Net cost 25 cash.