There is no certain way of getting Pearlescent Weapons on Borderlands. Pearlescent Weapons can be dropped by Crawmerax the Invincible but many people (including myself) have killed him lots of times with no luck. Drifters are said to drop Pearlescent Weapons sometimes and so are Crimson Lance Weapon Chests but the chances are minimal.
You can get them in any of the aforementioned ways they are rare but you'll have one appear at some point, i found a Maliwan Tsunami in a CL gun chest and a Jakobs Bessie after beating crawmerax solo (siren level 61)
I've killed Crawmerax maybe 15 times, no luck :/
If you just kill Crawmerax a lot, you'll get really good stuff, even if it's not Pearlescent, but you do have a chance.
It all boils down to luck and perserverance. My advice: Keep on killing every-damn-thing and opening Crimson Lance Chests. Best of luck --TGV--
All of the above ways are good, but no matter which one, it is probably ideal to have 4 people, as that makes loot a lot better. Also, if playing as Lilith, use a catalyst class mod, and Mordecai a scavenger class mod. - Nr DziNe