This item is only found in Lava Caverns, which is one of the last areas you'll be able to access. In Lava Caverns, there are only a few locations where you can find Iridescent Bismuth Prisms. So start searching from the northwest section of Lava Caverns.
You can also search during monster hunting, Master Rank or Expeditions. In the expeditions you will be free of worries, so you can search calmly. Just be careful of the hunts, so you don't get killed by any monster.
With prisms, you can craft or upgrade weapons and armor or use them to complete side quests.
Now that you know where to find Iridescent Bismuth Prisms in Monster Hunter Rise Sunbreak, you are ready to go for this item now that you know where you can find it, so best of luck on your search.