I don't think it's available yet in this update. I've done multiple playthroughs, trying to find hidden dialogues to unlock that outfit as well. Arvus official twitter said you can buy that outfit and the only place I think suitable for selling it is the Adult Store, yet there is none.
I even tried going to Adult Store after I dropped her off in the Mall since she said she want buy a few "things", but to no avail sadly.