Quote Originally Posted by douglas View Post
I haven't gotten far enough in the game to see for myself, but what I've read elsewhere is that there's a black market in Sanctuary that sells ammo and backpack SDUs (and also general high-end gear), and its prices are all in a special substance called Eridium that is a somewhat uncommon loot drop.
Crazy Earl has a black market shop that only takes Eridium and deals ammo upgrades and storage upgrades for increasing amounts of Eridium. You find him as a part of the quest line that introduces you to Sanctuary.