1. create a new world
2.type: flat world
3.go to customize the flat world
4 .use the presets "Tunnelers' Dream"
5.Igloos can appear
6.use the /locate command and it's going to work

example :

Seed : -1222959427155532512 in flat world with Tunnelers' Dream preset

an igloo will appear next to the spawn

Normally the structures that can have in the "Tunnelers' Dream" presets are strongholds, abandoned mineshafts and dungeons