I got a pop-up saying I can find it in missions, enemies, and treasure chests. Are all Eridium spawns random? Does the Eridium supply have a cutoff? Do certain chests or enemies in certain areas/locations always drop Eridium? Can I reduce the Eridium costs at Crazy Earl's/the Black Market to maximize the Eridium I do find? Where are the best locations to farm Eridium? What are the most time- and cost-effective methods for farming it?
According to the Borderlands 2 Wikia page related to Eridium:
So far it is unknown if there is a sure-fire way to obtain Eridium, but it is an uncommon-rare drop by certain enemies and rarely found in containers like lockers and washing machines. You can also obtain it by playing the slot machines at Moxxxi's place in Sanctuary.
Moxxxi's slot machines
According to this article the following is a list of potential payouts from playing the slot machines - in addition to randomly generated weapons and items of course;
- x3 Bandits: Live grenade (Run!)
- x3 7s: Customization skin
- x3 Cherries: Green item
- x3 Legs: Blue item
- x3 Marcus: Purple item
- x3 Borderlands logo: Orange item
- x3 Eridium Bar (1): 4 Eridium
- x3 Eridium Bars (2): 8 Eridium
- x3 Eridium Stack (3): 16 Eridium (!!!!)
- Any two with non-bell: White weapon
- Any two with bell at end: Cash
It is worth noting that playing the slot machines at Moxxxi's place will get more expensive as you progress through the game, yet some of the items you can win by playing the slots scale with your character - this means that playing the slots at a lower level will cost less while still potentially providing benefit to you at the later levels.
I have bought everything Crazy Earl has to offer and have then gone on to save a further 99 blocks of eridium. You cannot save any more than this. You can pick up the excess eridium, but it won't add to your tally.
Later on in the game, eridium drops become more common from defeated enemies. Earlier on you can scavenge plenty in Sanctuary, especially by checking inside the Crimson Raiders' HQ.
It can be found in lockers and cardboard boxes.