From Gamers Heroes: “Want to change outfits to DLC unlockables like the Idol outfits or swimsuits? This guide will tell you Where To Find DLC Outfits In Shining Resonance Refrain so you can enjoy the special outfits that came with the Japanese DLC, for free.”
Source: N4G PC Where To Find DLC Outfits In Shining Resonance Refrain