Yup, the only way to encounter Aero Spheres (and a bunch of other enemies with genomes) after Ch. 4 is through the bonus dungeon.
The quickest way you can unlock it is through the bad ending (Finale), during which you can speak to Lord DeRosso and he will open it for you. You can charm Aero Lanterns in there and get the White Wind genome. Be warned, though - the enemies you encounter are probably the most challenging in the entire game!
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