you can buy it at clothing vendor failing that it is found at the leather shop in fishtopia

You can definitely find it in random loot chests and it does have an item code. It's unfortunately just not in the wiki. There is an item code for it but I'm not sure what it is (most of the item codes are under 100 so you can try them 1 by 1 if you're really desperate). That's how I discovered the Martyr's armor set (item codes 50, 51, 52) and the bald hair style (99iirc) which aren't listed in the wiki. 

Also if you don't want Lona covering herself all game, just take the exhibitionist perk and she'll stick her arms up instead.