Iridium is a metal resource added by IC². It is not generated as ore, and must instead be found by exploration.
Iridium is most likely to be found in the Overworld (not in the Nether) as Iridium Shards, which can then be compressed to form Iridium Ore. Iridium can also sometimes be found in End Cities. The ore can be macerated back into shards if needed, with no loss of material.
Due to Iridium's rarity, it is highly recommended to scan it for later replication. (Unless you want to run around the world for literal days hunting down enough, in which case - have fun!)