These can be obtained by buying the collectible maps from Two-Hat Jack, and then collecting them in Sunset City. Once you have completed the story and challenges it is extremely hard to get any overcharge for it. You don't need the 100k weapon, I bought it and I struggled getting overcharge, so buy the maps first! If like me however, you are finding it hard to get overcharge later on in the game, this is the method I used to grind overcharge quite quickly to help me get the last maps I needed.
This setup will help speed things up, to get the most of your time:
Weapon - Hair Spray Bomb with SECOND AMENDMENT amp (this allows the weapon to keep regenerating ammo so you never have to use anything else)
Electric damage (only use this if you use the Chance of Lightning Epic Amp)
Sacrifice single shot (50% more damage with the hairspray bomb)
Load up Mission A heros duty and follow the video below :)