according to borderlands wiki, pearlescent weapons are exclusive to third borderlands DLC, if you don't have it that's probably your problem, if so, it goes on to say, "They can be found in weapon boxes, dropped by Crawmerax the Invincible, bosses during This ***** is Payback, and less commonly by Lancers. They can also be dropped by BadAss Bruisers, Lance Pyros, Badass Engineers, Drifters and on rare occasion can be found by opening Crimson Lance chests."
(A fairly low level one though):
And like said before in this thread,
you have to have the General Knoxx DLC to be able to get them.
Oh, they have a much,
much bigger chance of dropping in a 4 player co-op game.
[Edited by malmern, 10/11/2010 2:25:04 AM]