Gold Pine Resin is a weapon buff consumable that will add about 150 Lightning Damage to your right hand weapon. The buff will last 1 minute or until the weapon is switched out.
You can pick up 3x Gold Pine Resin from a chest in Undead Burg if you have the Residence Key. Domhnall of Zena will sell you three batches and Elizabeth in Oolacile Sanctuary will sell you five batches. The cost is 1,000 from both.
Alternatively, you can farm it from large Mushroom People. Gold Pine Resin is not a guaranteed drop, but it is common. The resin is a rare drop from the smaller Mushroom People, so don't bother with them. The two large Mushroom People in Darkroot Garden are a very good choice for farming.
One of the large Mushroom People in the Darkroot Garden. A good source of Gold Pine Resin.