If your Illia is suffering from a severe case of Where-in-Oblivion-is-Carmen-Sandiego-itis (She's nowhere to be found), please try entering Darklight Tower, standing in the room where you first encountered her and waiting for 1-3 hours.
All jokes aside, I read Illia's entire page on this site and every forum post with her as a topic. No one suggested this. I tried following the entire Clairvoyance path from Winterhold (the area I dismissed her) to Darklight Tower. I found nothing, even casting Detect Life/Dead the whole way, and it took a long time. I decided to share my successful endeavor (It happened about fifteen minutes prior to posting this).
Please note that I'm playing the Xbox 360 version of "Legendary Edition" (DLC bundle) not "Special Edition" (New-Gen Remaster). Why such an old version? I'm poor and the newest console I have is a Wii U. Hope this method will help some other ignorant previous-save-less console peasant like myself. Pray to Zenithar for me.