You will get this when upgrading weapons and Vigors. If you reload a checkpoint, this will continue to count. You can grind this out with reloading this way. Just spend a few thousand in Vigors/Weapons vending machines then reload your checkpoint as long as there is one close by.
You get money for killing people, looting corpses, and containers. You will get this in the process of upgrading multiple vigors and weapons as well as buying items and ammo through play. It should unlock around halfway through the game.
Note: Remember if you are on 1999 Mode, do not purchase from the Dollar Bill vending machines.