White Pearl Loop, also known as Pearl Necklace in the US, can be purchased from stores that sell Uncommon treasures. (Uncommon treasures are White/Dark Pearl Loop, Pirate's Necklace, Ruto Crown and Dragon Scale.) Those stores are located in Castle Town (two random uncommon treasures per day) and Snowfall Sanctuary (one uncommon treasure per day). You can also buy them in Beedle's shop balloon, where there is an uncommon treasure and a rare one for sale each day. Some minigames (Whip Race, Goron Shooting Range, Pirate Hideout) award Uncommon treasures for a mediocre performance.
If you bring the builder to penguin town he'll give you e pearl loop
Oh, if only I knew. Stupid iron freight car is driving me nuts with it's taunts :9
Pearl Loops can be found easily by doing the whip race in Whittleton under one minute and fifteen seconds. It gives out uncommon treasures. I got five in twelve minutes.