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?