Pokémon Ultra Sun & Ultra Moon - Ultra Space

Is there a way to increase the chance of finding Legendaries? - PokéBase Pokémon Answers

Ultra Space Guide - Pokemon Ultra Sun and Ultra Moon

What's New In Pokemon Ultra Moon And Sun: Ultra Beasts, Extra Pokemon, Z-Moves, Fresh Features, And More - GameSpot

Pokémon Ultra Sun & Ultra Moon - Ultra Wormholes

Ultra Space Wilds - Bulbapedia, the community-driven Pokémon encyclopedia

Ultra Sun & Moon Theory: Ultra Space is Canon!?

Guide to Ultra Space in Pokémon Ultra Sun and Moon - Smogon University

Pokémon Ultra Sun & Ultra Moon - Ultra Wormholes

Pokémon UltraSun & UltraMoon - Ultra Space Wilds Music (HQ)

What Pokémon: Ultra Sun and Ultra Moon Got Wrong, by Leon Barillaro

Pokemon Trainer in the Ultra Space of Madness (2017) : r/pokemon

Ultra Space was apparently one of the concept art sheets shown at Milan Games Week. It looks like it also has some interesting notes about Nihilego. : r/pokemon