Eridium is part of the world drop, so there’s no specific location for it. Generally, you’ll find either eridium, cash, or health thingies in the various types of cash box, as well as lockers, chests, fridges, and washing machines. You can also have it drop from random enemies or pop out of the various poop piles around the game.
If you’re specifically trying to get eridium to max your storage and ammo slots, most people farm either the Warrior, the Handsome Sorcerer, or the Ancient Dragons. Note that you can farm them on any mode (eg normal) and you will still have the eridium when you restart in a higher mode (eg UVHM).