Why have there been so many stupid questions here lately? Pearlescent (as well as any other non unique item) are dropped randomly. Pearlescents can only be found in DLC3, either in Crimson Lance Weapon Chests and from Badass bandits and Drifters as well as Skyscraper.
EDIT: Unless you're on PC and want to install a custom map, none; Random loot is random. Crawmerax is your best bet otherwise.
EDIT: Perlescent weapons are a completely random drop, there is no predefined location.
EDIT: They drop randomly. Try farming crawmerax.