Small Fishing Net is a consumable on FarmVille which is used for opening Small and Medium 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 Small Fishing Net. There is also other packs with more this consumables - pack of 5 Small Fishing Net cost 5 cash and pack of 10 Small Fishing Net cost 10 cash.