These upgrades are useful and the only way to get them is through the Black Market in Sanctuary using Eridium. There are other upgrades too: bank slots, backpack slots, max grenades, etc. In Borderlands 2, Eridium (not Iridium!) is a currency (similar to Seraph Crystals), that lets you purchase special upgrades for Ammo SDU (storage deck upgrade), thus allowing you to hold a larger ammo capacity for each gun type you choose.