Chubby can be found all around Pandora, but there's two good spots you're guaranteed to find one and that is in the starting area of the Caustic Caverns (Cubby Vakid) & the Wildlife Exploration Preserve (Cubby Stalker). It has a fixed spawn point where the first group of enemies are located, right next to the fast travel stations. So you can back out to the main menu and hit continue to make all enemies respawn but take note they're ultra rare, so it may take some time. A Chubby is not a normal enemy, he's a variation of an enemy type, so - Varkid, Spiderant, Skag, Stalker, Midget and Rakk.

For a list of other known locations, please see below.

Blake Bridge
Underground area's of Frostburn Canyon
The Dust
Firestone
Slaughter Dome