Borderlands 1
Rain

Base Game - World Drop

Borderlands 1 World Drops are items that drop from any suitable Loot Source, like Bosses, regular enemies, red-chests, white-chests, and vending machines.
With 9 grenades in the World Drop pool, getting 1 specific grenade is difficult.

Awesome Level

Each Loot Sources has an Awesome Level that determines a rarity's weighting in the World Drop pool.
The higher the Awesome Level the higher is the weighting of Rare, Epic, and Legendary loot.

Lilith's Catalyst and Mordecai's Scavenger Class Mod can increase the Team's Awesome Level by an insignificant amount. But it's notable for low Awesome Level Sources like chests and generic enemies.
Best Loot Sources are:

• Crawmerax
• Crimson Armory
• Destroyer Chest
• Robot Revolution - Loot Room