On the Rain-Slick Precipice of Darkness/Machine Parts

As you explore New Arcadia, you will collect Machine Parts, both from felled Fruit Fucker enemies and sometimes just lying around. There are enough Machine Parts to upgrade all three characters' weapons to their third and final "normal" iteration.

Desperation St
All the Machine Parts in Desperation Street will be collected automatically by defeating all the enemies encountered during the Tutorial.
 * Total parts: 19

Hobo Alley

 * Total parts: 122

The Shithole Warehouse
There are spare parts lying on all the tables in the warehouse. Inspect them all to pick up the parts.

Pelican Bay
The Fruit Fucker (Mark IV) enemies in this level drop around 25 parts each when defeated.
 * Total parts: 450