Found initially at Land's End, Tidal Pools are the only source of Pearls. They also provide Clams on occassion when tended. Although treated as a pool, (eg as in the Traveler from Afar Quest 2 of 3), they never provide pails of water to the owner, though allies can receive this. They can be tended every 5 minutes.
One is available as a quest reward for the 3rd Land's End quest. After this, they can be bought for 19000 coins in the market.