>>715548979
I'm not sure if you have to rank up your guild rank for the store to be open (since the rank up quest was what sent me down here) but there was a shop here with a whole plethora of new outfits. I then had to go to the desert town after which had its own lineup of outfits.
When you get past guild rank E you start getting side quests that have outfits as rewards. You may have to keep playing the story until Sister and Witch join to be able to level that up. Keep in mind there's just 10 story chapters anyway.
The outfit in the OP was from a character quest with Hermit and Witch.

>>715399095
>>715399232
>>715399383
PSA btw I found just these outfits alone + like 7 more down here at this shop.