FRP, or Fiber Reinforced Plastic, can be acquired by using a Shovel on the island directly left of the main Island.
FRP is used in the following recipes:
Where to FindEdit
Once you have a Raft go to the island directly left of the main Island. Similar to the Iron Ingot, FRP is obtained by using a shovel on a hole under a Potato bush on this island. The hole looks identical to the one the Iron Ingot was in so it should not be a problem to find it once you are in this area. It does not respawn.