You can get Wildcards through the Early Ascension of Celestial Monsters on Celestial Island. If an early ascension attempt fails when a Celestial Monster’s inventory is nearly full, you can earn Wildcards. Wildcards cannot be obtained if the Celestial has successfully ascended, its inventory is full, or if the time to ascend has expired.

Wildcards can also be collected from the Nexus Nucleus on Magical Nexus.