Iridium ore in the Skull Cavern in the Desert becomes more common the deeper you go. It should become a regular drop around level 70. Purple Slimes, Bats and Crabs have a chance to drop Iridium when killed. You can find all of these in the Skull Cavern.

How do you spawn iridium in Stardew Valley?

Iridium Ore Spawn Help

To spawn this item using an animal's name, visit Marnie's Ranch, speak to Marnie and purchase an animal (we recommend a chicken as it is cheapest). Name the animal [386] and press OK. You will then receive the iridium ore item.

How do I get more Iridium?

Iridium can be found and mined in the Skull Cavern in the desert. Once you have reached the bottom of the mines and fixed the bus, go to the cave in the top-left of the desert. The enemies there are tough but you're much more likely to find iridium and there's more of it the deeper you go.