Use the interactive seed-map finder on this page: enter your world seed and it plots the exact coordinates of every igloo in your world. Travel to those X/Z coordinates in any snowy biome, and look for the small snow-block hut with a carpet inside that may hide a basement.
Igloos generate only in snowy Overworld biomes: Snowy Plains, Snowy Taiga, and (rarely) Snowy Slopes. They are surface structures, so you will see the little snow-and-ice hut sitting on top of the snow.
About half of all igloos hide a basement reached by a trapdoor under the carpet. It contains a brewing stand, a captive Villager and Zombie Villager (so you can cure the zombie for very cheap trades), and a loot chest that can hold a golden apple along with emeralds, gold nuggets, and other supplies.
