Limit the range of the search, be realistic with the file size (typical costume ranges hovers around 2000 KB). Copy all those within your particular range with an amplitude of ~1000 KB. Alternatively, if you have another original client's file, use the windows search bar and type "size: xxKB" this will throw up all files with the exact same file size.
Copy and Paste it into a sandbox folder, preferably within Local\NCxxxx\xxYourLanguage\ folder path (in my case Local\NCTAIWAN\CHINESET\Sandbox_LynF), unsure if it's just my case however this is required for umodel to work properly.
Now we filter out all the other files from the textures.
First remove any files that contains letters. Then by using either umodel put your sandbox's folder path, override game detection to UE3 and Blade & Souls, uncheck everything besides textures. To open each file press Enter, to go back to the overview press O, and in the overview press down arrow to go to the proceeding file. Sift through each file using the aforementioned keyboard hotkey method, it goes much quicker this way. If any of the upks contain the word "LynF" note the file name within notepad. Also keep mind there may be other files that look like body textures without proper syntax and only include _s, _d, _m; these files I suspect are files that were altered and resaved, as such it may be good idea to note those numbers down as well.
Adding compatibility (?? pretty sure there's a better way to go about doing this, so I won't be putting instructions down for this).
Hope this helps, and if anyone have anything else to add, you're more than welcome to.